IPIP: Streaming Delegated Content Routing

[guseggert]

To support DHT use cases, the Delegated Content Routing HTTP API needs to support a streaming response that streams providers as they are found, probably using an application/x-ndjson response format.

[guseggert]

This is implemented and tested in ipfs/boxo#18.

A note on errors:

To match the existing content routing interface (which does not support any sort of errors), the schema does not allow for passing per-record errors. The schema does allow for adding per-record errors to the struct if a future use-case arises for that.

If there is an unrecoverable error before streaming begins, then the server can send an HTTP error status code, otherwise if the server has started streaming the response and encounters an unrecoverable error, then it must terminate the connection to signal an error.