Implement selections Beacon API endpoints to support DVT middleware

[chong-he]

Issue Addressed

• DVT Compatibility #6610

Proposed Changes

• Add beacon_committee_selections endpoint
• Test beacon committee aggregator and confirmed working
• Add sync_committee_selections endpoint
• Test sync committee aggregator and confirmed working

Additional Info

Thank you @michaelsproul and @macladson for the help and guidance

[chong-he]

With the commit 8b2f058 where VC sends partial signatures in parallel for all validators for a slot, I believe the aggregated attestations are successful (during testing) now, thank you @michaelsproul !

Lighthouse VC logs:

{"log":"Mar 20 06:36:24.046 INFO Successfully published attestation      type: aggregated, slot: 37, committee_index: 0, head_block: 0xdb186315afecb3d1134d03eb2d5b04b83a21ede56be6563e3bff304b103bf4b6, signatures: 24, aggregator: 58, service: attestation\n","stream":"stderr","time":"2025-03-20T06:36:24.04690072Z"}

for every slot now

Charon logs:

{"log":"06:36:24.074 INFO bcast      Successfully submitted attestation aggregations to beacon node {\"delay\": \"8.074945477s\", \"duty\": \"37/aggregator\", \"peer\": \"twinkling-pen\", \"protocol\": \"/charon/parsigex/2.0.0\"}\n","stream":"stderr","time":"2025-03-20T06:36:24.074999639Z"}

for every slot now too

@KaloyanTanev could you give it a try and confirm that the beacon committee selection endpoint is working as intended?

Thanks

[dknopik]

Thanks! I've added a few notes.

[mergify]

This pull request has merge conflicts. Could you please resolve them @chong-he? 🙏

[chong-he]

Thanks! I've added a few notes.

Thank you so much for the review and help. All revised accordingly, let me know if further changes required.

[dknopik]

Do you know why this is needed (here and above)? It seems weird to me to compute the proof for the previous slot.

[chong-he]

I honestly don't know why. @michaelsproul might have an answer for this.

In the distributed mode, I follow the same logic to also compute slot - 1