feat(json-rpc): add beacon network JSON-RPC endpoints

[ogenev]

Add basic beacon network endpoints to JSON_RPC specs.

All of the endpoints are copy/paste from the history network except this one:

• I'm proposing adding portal_beaconOptimisticStateRoot endpoint that returns the hex encoded recent known beacon state root.

[acolytec3]

• I'm proposing adding portal_beaconOptimisticStateRoot endpoint that returns the hex encoded recent known beacon state root.

To confirm, this new one is just returning the beacon state root embedded in the optimisticUpdate that we have stored locally?

[ogenev]

To confirm, this new one is just returning the beacon state root embedded in the optimisticUpdate that we have stored locally?

yes

[ogenev]

Thinking about the state root endpoints more, we will want also similar endpoint for the finality updates: portal_beaconFinalityStateRoot and probably adding additional field to the response object, the slot number?

Current use cases:

• testing if a portal node is in sync in Hive - check if latest optimistic and finality roots/slots match the expected.
• Glados monitoring - check if the updates are available on the network based on the expected slot number.