Add custody_group_count to metadata in node/identity response

[jimmygchen]

Add custody_group_count to metadata.

This field is part of MetaDataV3 used by PeerDAS and will be available from the Fulu fork.

https://github.com/ethereum/consensus-specs/blob/e79ef816aef05e34eff0b53311cf50c13fb67fc9/specs/fulu/p2p-interface.md?plain=1#L159

[nflaig]

I guess we did the same for syncnets in #150 to add it as an optional field.

The problem with this approach is that it makes returning a ssz response more complicated, although as per spec this api only supports json.

I would be in favor of bumping the api version if we add new properties to data object but this seems reasonable as well since we used the same approach before.

[rolfyone]

LGTM

[dapplion]

The problem with this approach is that it makes returning a ssz response more complicated, although as per spec this api only supports json.

For SSZ you could set both values to zero if pre-v3

[nflaig]

For SSZ you could set both values to zero if pre-v3

That works if all clients do this but without version bump of the api you don't really know what to expect and might get different result based on client version.

My comment was more general that we shouldn't add optional fields, /node apis already don't work well with SSZ anyways.

[agnxsh]

lgtm