Add blob_sidecar event to SSE

[jimmygchen]

Issue Addressed

Partially address #4722 (as the issue also covers potential block event changes in Deneb, which aren't covered in this PR)

Proposed Changes

• Introduce a BlobSidecar SSE event, and register the event when a the beacon node has received a BlobSidecar from either P2P or API)
  • For blobs received via gossip or beacon API (publish block), event are emitted for each blob that passes gossip validations.
  • For blobs received via a RPC request, events are emitted once they're received and sent over to the BeaconChain for processing.
• An unrelated fix (could move it in a separate PR): return a success response code (202) if validation level is gossip and blob validation fails after the block has successfully broadcasted to the network (this is consistent with block validation behaviour).

[jimmygchen]

Had a bit of discussion with @pawanjay176 on this today, and I think I might park this for a bit, as there are some ongoing discussions on when to emit the block events, and may potentially impact blob events (on whether to emit it after gossip or after full validation).

If we're doing it after gossip, Pawan suggested

we can then do it in BeaconChain::process_gossip_blob for blobs received over gossip

This function is new in @realbigsean 's PR and hasn't landed on Deneb branch yet, so seems to make sense to wait for a bit.

On the testing side it doesn't seem too complicated, we could potentially pass a GossipVerifiedBlob to process_gossip_blob and assert that subscribers of ServerSentEventHandler receives an event.

[jimmygchen]

I've merged latest deneb-free-blobs into this branch (given #4732 was merged), and updated the BlobSidecar event to be spec compliant.

[ethDreamer]

LGTM!

[ethDreamer]

Nice work! You don't have to do this but since you seeded the RNG with a known value, the blob generated should be the same each time. You could check that the output matches the static SseBlobSidecar here so that this test breaks if something goes wrong in the code we use to generate it. Just an idea.

[jimmygchen]

Good call! Addressed in 39cc2fe. Thanks 🙏

[realbigsean]

Think we just need to add quotes on the JSON representation of a blob index. But nice tests! And also nice bonus fix on the blob gossip without import.