Add broadcast validation routes to Beacon Node HTTP API (sigp#4316)

 - sigp#4293
 - sigp#4264

*Changes largely follow those suggested in the main issue*.

 - Add new routes to HTTP API
   - `post_beacon_blocks_v2`
   - `post_blinded_beacon_blocks_v2`
 - Add new routes to `BeaconNodeHttpClient`
   - `post_beacon_blocks_v2`
   - `post_blinded_beacon_blocks_v2`
 - Define new Eth2 common types
   - `BroadcastValidation`, enum representing the level of validation to apply to blocks prior to broadcast
   - `BroadcastValidationQuery`, the corresponding HTTP query string type for the above type
 - ~~Define `_checked` variants of both `publish_block` and `publish_blinded_block` that enforce a validation level at a type level~~
 - Add interactive tests to the `bn_http_api_tests` test target covering each validation level (to their own test module, `broadcast_validation_tests`)
   - `beacon/blocks`
       - `broadcast_validation=gossip`
         - Invalid (400)
         - Full Pass (200)
         - Partial Pass (202)
        - `broadcast_validation=consensus`
          - Invalid (400)
          - Only gossip (400)
          - Only consensus pass (i.e., equivocates) (200)
          - Full pass (200)
        - `broadcast_validation=consensus_and_equivocation`
          - Invalid (400)
          - Invalid due to early equivocation (400)
          - Only gossip (400)
          - Only consensus (400)
          - Pass (200)
   - `beacon/blinded_blocks`
       - `broadcast_validation=gossip`
         - Invalid (400)
         - Full Pass (200)
         - Partial Pass (202)
        - `broadcast_validation=consensus`
          - Invalid (400)
          - Only gossip (400)
          - ~~Only consensus pass (i.e., equivocates) (200)~~
          - Full pass (200)
        - `broadcast_validation=consensus_and_equivocation`
          - Invalid (400)
          - Invalid due to early equivocation (400)
          - Only gossip (400)
          - Only consensus (400)
          - Pass (200)
 - Add a new trait, `IntoGossipVerifiedBlock`, which allows type-level guarantees to be made as to gossip validity
 - Modify the structure of the `ObservedBlockProducers` cache from a `(slot, validator_index)` mapping to a `((slot, validator_index), block_root)` mapping
 - Modify `ObservedBlockProducers::proposer_has_been_observed` to return a `SeenBlock` rather than a boolean on success
 - Punish gossip peer (low) for submitting equivocating blocks
 - Rename `BlockError::SlashablePublish` to `BlockError::SlashableProposal`

This PR contains changes that directly modify how blocks are verified within the client. For more context, consult [comments in-thread](sigp#4316 (comment)).

Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/beacon_chain/src/builder.rs

@@ -820,7 +820,6 @@ where
             observed_sync_aggregators: <_>::default(),
             // TODO: allow for persisting and loading the pool from disk.
             observed_block_producers: <_>::default(),
-            // TODO: allow for persisting and loading the pool from disk.
             observed_voluntary_exits: <_>::default(),
             observed_proposer_slashings: <_>::default(),
             observed_attester_slashings: <_>::default(),

beacon_node/beacon_chain/src/lib.rs

@@ -65,7 +65,7 @@ pub use attestation_verification::Error as AttestationError;
 pub use beacon_fork_choice_store::{BeaconForkChoiceStore, Error as ForkChoiceStoreError};
 pub use block_verification::{
     get_block_root, BlockError, ExecutionPayloadError, GossipVerifiedBlock,
-    IntoExecutionPendingBlock,
+    IntoExecutionPendingBlock, IntoGossipVerifiedBlock,
 };
 pub use canonical_head::{CachedHead, CanonicalHead, CanonicalHeadRwLock};
 pub use eth1_chain::{Eth1Chain, Eth1ChainBackend};