add /eth/v2/beacon/blinded_blocks to validatorapi

[xenowits]

🎯 Problem to be solved

A new v2 endpoint to submit blinded beacon blocks to a beacon node has been recently added to the beacon API specs. We need to support newer versions of validator clients quering this v2 endpoint.

🛠️ Proposed solution

Add support for the new /eth/v2/beacon/blinded_blocks endpoint to the validatorapi component.

🧪 Tests

• Tested by new automated unit/integration/smoke tests
• Manually tested on core team/canary/test clusters
• Manually tested on local compose simnet

[corverroos]

@xenowits do we know of any VCs that use this endpoint?

[xenowits]

@corverroos No, none of the VCs seem to use this v2 endpoint. The following VCs are all using v1 endpoint to submit blinded beacon blocks atm:
Teku: https://github.com/Consensys/teku/blob/22.12.0/data/beaconrestapi/src/main/java/tech/pegasys/teku/beaconrestapi/handlers/v1/beacon/PostBlindedBlock.java#L42
Lighthouse: https://github.com/sigp/lighthouse/blob/stable/validator_client/src/block_service.rs#L572
Lodestar: https://github.com/ChainSafe/lodestar/blob/unstable/packages/validator/src/services/block.ts#L187
Nimbus: https://github.com/status-im/nimbus-eth2/blob/stable/beacon_chain/validator_client/block_service.nim#L252

[dB2510]

Closing this as it is completed as part of #2650