Caplin: beacon committee subscription api

[domiwei]

This pr implements following beacon api
https://ethereum.github.io/beacon-APIs/#/Validator/prepareBeaconCommitteeSubnet
Steps:

1. once receive committee subscription request, record this subscription info associated with validator index.
2. if isAggregator flag is on, prepare internal data structure to collect and aggregate incoming attestation.
3. set expiry and begin to subscribe beacon_attestation_{subnet}, so we are capable of receiving attestation data from peer.
4. whenever receive attestation data, try aggregate it if needed.

[Giulio2002]

General feedback: Maybe for aggregating signatures and public key it is better to do another AttestationAggregator struct. the reason is that we can get that from different sources such as: aggregates topic, attestation topics or from the beacon API.

[sonarcloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud