Implement RPC endpoints required by beaconcha.in explorer

[corduroybera]

Endpoint calls referenced in beaconcha.in repo:

• eth/v1/config/spec - chore(node-api): Implement config spec API #2056
• eth/v1/beacon/headers/ - test(node-api) : Beacon headers and genesis API  #2002
• eth/v1/beacon/headers/{block_id} - test(node-api) : Beacon headers and genesis API  #2002
• eth/v1/beacon/blob_sidecars/{block_id} - chore(node-api): stub for getting blob sidecars  #2064 stub - real implementation TODO
• eth/v1/beacon/genesis - test(node-api) : Beacon headers and genesis API  #2002 and feat(node-api) : Implemented genesis time  #2014
• eth/v1/beacon/pool/bls_to_execution_changes - TODO
• eth/v1/events?topics=head - TODO
• eth/v1/beacon/states/{state_id}/finality_checkpoints - TODO
• eth/v1/beacon/states/{state_id}/validators - test(node-api): Introduce E2E tests for existing beacon endpoints #1983 and chore(node-api) : Get Validators by State ID  #2050
• eth/v1/validator/duties/proposer/{epoch} - TODO
• eth/v1/beacon/states/{state_id}/committees?epoch=%d - TODO
• eth/v1/beacon/states/{state_id}/validator_balances - test(node-api): Introduce E2E tests for existing beacon endpoints #1983
• eth/v1/beacon/states/genesis/validator_balances - test(node-api): Introduce E2E tests for existing beacon endpoints #1983
• eth/v1/beacon/states/{state_id}/sync_committees - TODO
• eth/v1/beacon/pool/voluntary_exits - TODO
• eth/v2/beacon/blocks/{block_id} - TODO