Implement basic validator custody framework (no backfill)

[pawanjay176]

Issue Addressed

Resolves #6767

Proposed Changes

This PR implements a basic version of validator custody.

• It introduces a new CustodyContext object which contains info regarding number of validators attached to a node and the custody count they contribute to the cgc.
• The CustodyContext is added in the da_checker and has methods for returning the current cgc and the number of columns to sample at head. Note that the logic for returning the cgc existed previously in the network globals.
• To estimate the number of validators attached, we use the beacon_committee_subscriptions endpoint. This might overestimate the number of validators actually publishing attestations from the node in the case of multi BN setups. We could also potentially use the publish_attestations endpoint to get a more conservative estimate at a later point.
• Anytime there's a change in the custody_group_count due to addition/removal of validators, the custody context should send an event on a broadcast channnel. The only subscriber for the channel exists in the network service which simply subscribes to more subnets. There can be additional subscribers in sync that will start a backfill once the cgc changes.

TODO

• NOT REQUIRED: Currently, the logic only handles an increase in validator count and does not handle a decrease. We should ideally unsubscribe from subnets when the cgc has decreased.
• NOT REQUIRED: Add a service in the CustodyContext that emits an event once MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS passes after updating the current cgc. This event should be picked up by a subscriber which updates the enr and metadata.
• Add more tests

[dknopik]

Took a look at this as an opportunity to refamiliarise myself with the DAS code and spec. I hope I looked at the correct spec version x)

Also, just noting as I found no explicit TODO or code for this: I think we also need to update the ValidatorRegistrations if the effective balance changes due to beacon chain rewards, penalties, deposits and (partial) withdrawals.

[jimmygchen]

LGTM

Next changes (separate PRs)

• add lowest block slot to status message, update it on CGC change, & use lowest block slot for peer selection when making rpc requests
• use additional http api to register validators
• backfill

[pawanjay176]

LGTM as well. Thanks for fixing up most of the outstanding issues!

[mergify]

Some required checks have failed. Could you please take a look @pawanjay176? 🙏