Reintroduce provide records via PUT to HTTP delegated routing

Previous work reduced the scope of IPIP-337 to read operations only. The
changes here re-introduce the write operations originally written by
@guseggert back into IPIP-337.

The specification documents the ability to provide Bitswap records over
`PUT` requests with advisory TTL. An implementation of the specification
is already present in go-libipfs.

See:
 - #370

routing/DELEGATED_CONTENT_ROUTING_HTTP.md

@@ -90,6 +90,39 @@ Response limit: 100 providers
 
 Each object in the `Providers` list is a *read provider record*.
 
+### `PUT /routing/v1/providers`
+- Response Codes
+  - `200`: the server processed the full list of provider records (possibly unsuccessfully, depending on the semantics of the particular records)
+- Request Body
+```json
+{
+    "Providers": [
+        {
+            "Protocol": "<protocol_name>",
+            "Schema": "bitswap",
+            ...
+        }
+    ]
+}
+```
+
+Each object in the `Providers` list is a *write provider record*.
+
+- Response Body
+```json
+{
+    "ProvideResults": [
+        { ... }
+    ]
+}
+```
+- `ProvideResults` is a list of results in the same order as the `Providers` in the request, and the schema of each object is determined by the `Protocol` of the corresponding write object (called "Write Provider Records Response" in the Known Transfer Protocols section)
+  - This may contain output information such as TTLs, errors, etc.
+  - It is undefined whether the server will allow partial results
+- The work for processing each provider record should be idempotent so that it can be retried without excessive cost in the case of full or partial failure of the request
+- Default limit of 100 keys per request
+- Implements pagination according to the Pagination section
+
 ## Pagination
 
 This API does not support pagination, but optional pagination can be added in a backwards-compatible spec update.
@@ -118,7 +151,7 @@ limits, allowing every site to query the API for results:
 
 ```plaintext
 Access-Control-Allow-Origin: *
-Access-Control-Allow-Methods: GET, OPTIONS
+Access-Control-Allow-Methods: GET, PUT, OPTIONS
 ```
 
 ## Known Transfer Protocols
@@ -148,6 +181,59 @@ Specification: [ipfs/specs/BITSWAP.md](https://github.com/ipfs/specs/blob/main/B
 
 The server should respect a passed `transport` query parameter by filtering against the `Addrs` list.
 
+
+#### Bitswap Write Provider Records
+
+```json
+{
+    "Protocol": "transport-bitswap",
+    "Schema": "bitswap",
+    "Signature": "<signature>",
+    "Payload": "<payload>"
+}
+```
+
+- `Signature`: a multibase-encoded signature of the sha256 hash of the `Payload` field, signed using the private key of the Peer ID specified in the `Payload` JSON. Signing details for specific key types should follow [libp2p/peerid specs](https://github.com/libp2p/specs/blob/master/peer-ids/peer-ids.md#key-types), unless stated otherwise.
+
+  - Servers may ignore this field if they do not require signature verification.
+- `Payload`: a string containing a serialized JSON object which conforms with the following schema:
+   ```json
+   {
+       "Keys": ["cid1", "cid2"],
+       "Timestamp": 0,
+       "AdvisoryTTL": 0,
+       "ID": "12D3K...",
+       "Addrs": ["/ip4/..."]
+   }
+   ```
+- `Keys` is a list of the CIDs being provided
+- `Timestamp` is the current time
+- `AdvisoryTTL` is the time by which the caller expects the server to keep the record available
+  - If this value is unknown, the caller may use a value of 0
+- `ID` is the peer ID that was used to sign the record
+- `Addrs` is a list of string-encoded multiaddrs
+
+A [400 Bad Request](https://httpwg.org/specs/rfc9110.html#status.400)  response code should be returned if the signature check fails.
+
+Note that this only supports Peer IDs expressed as identity multihashes. Peer IDs with older key types that exceed 42 bytes are not verifiable since they only contain a hash of the key, not the key itself.
+Normally, if the Peer ID contains only a hash of the key, then the key is obtained out-of-band (e.g. by fetching the block via IPFS).
+If support for these Peer IDs is needed in the future, this spec can be updated to allow the client to provide the key and key type out-of-band by adding optional `PublicKey` and `PublicKeyType` fields, and if the Peer ID is a CID, then the server can verify the public key's authenticity against the CID, and then proceed with the rest of the verification scheme.
+
+The `Payload` field is a string, not a proper JSON object, to prevent its contents from being accidentally parsed and re-encoded by intermediaries, which may change the order of JSON fields and thus cause the record to fail validation.
+
+#### Write Provider Records Response
+
+```json
+{
+    "AdvisoryTTL": 0
+}
+```
+
+- `AdvisoryTTL` is the time at which the server expects itself to drop the record
+  - If less than the `AdvisoryTTL` in the request, then the client should re-issue the request by that point
+  - If greater than the `AdvisoryTTL` in the request, then the server expects the client to be responsible for the content for up to that amount of time (TODO: this is ambiguous)
+  - If 0, the server makes no claims about the lifetime of the record
+
 ### Filecoin Graphsync
 
 Multicodec name: `transport-graphsync-filecoinv1`
@@ -173,3 +259,7 @@ Specification: [ipfs/go-graphsync/blob/main/docs/architecture.md](https://github
 - `PieceCID`: the CID of the [piece](https://spec.filecoin.io/systems/filecoin_files/piece/#section-systems.filecoin_files.piece) within which the data is stored
 - `VerifiedDeal`: whether the deal corresponding to the data is verified
 - `FastRetrieval`: whether the provider claims there is an unsealed copy of the data available for fast retrieval
+
+#### Filecoin Graphsync Write Provider Records
+
+There is currently no specified schema.