Update beacon API PR

[xenowits]

Problem to be solved

We raised a PR couple of months back proposing a new (v2) version of /eth/v1/validator/beacon_committee_subscriptions. Since then, we have made progress and have thought deeply on how best to tackle the issue of committee aggregations in a middleware DVT architecture such as charon's.

We have come up with a new proposal to include two NEW endpoints for committee aggregations (beacon committee and sync committee).

POST /eth/v1/validator/beacon_committee_selections
POST /eth/v1/validator/sync_committee_selections

Proposed solution

Update the EXISTING PR with the new proposal and proper explanation of our approach. We are refraining to create a new PR as the current one already has got a lot of useful feedback from some important people which we would not want to lose.

Also, write an https://ethresear.ch/t/ article explaining the idea.

Out of Scope

None.