Add execution_payload_gossip event

[nflaig]

This adds execution_payload_gossip to the eventstream.

The event mirrors the existing block_gossip.

In contrast to execution_payload_available, this event is emitted based on the execution payload only and does not require full data availability.

This event is required to measure timings of execution payloads via sidecar software such as contributoor.

[rolfyone]

we should probably add versions to this too...

[nflaig]

we should probably add versions to this too...

was thinking about that, I am not that convinced that versioning here makes sense as the schema is not based on a spec container

[rolfyone]

was thinking about that, I am not that convinced that versioning here makes sense as the schema is not based on a spec container

still, we might be better off being consistent?

[nflaig]

still, we might be better off being consistent?

but that means not having the version/data container here? eg. block_gossip doesn't have that either because the event doesn't emit data based on a spec container while evens in #587 do, so having the version there makes sense

I am open to adding a version here too if that's really what we want

[rolfyone]

@michaelsproul thoughts?