Add selections API endpoints to support DVT middleware

[chong-he]

Issue Addressed

• DVT Compatibility #6610

Proposed Changes

• Add beacon_committee_selections endpoint
• Test beacon committee aggregator
• Add sync_committee_selections endpoint
• Test sync committee aggregator

Additional Info

Thank you @michaelsproul and @macladson for the help and guidance

[michaelsproul]

This line is saying that we expect a Vec<SelectionProof> as the response. A SelectionProof is just a standalone signature. But the middleware will actually return a Vec of objects like this:

{
  "data": [
    {
      "validator_index": "1",
      "slot": "1",
      "selection_proof": "0x1b66ac1fb663c9bc59509846d6ec05345bd908eda73e670af888da41af171505cc411d61252fb6cb3fa0017b679f8bb2305b26a285fa2737f175668d0dff91cc1b66ac1fb663c9bc59509846d6ec05345bd908eda73e670af888da41af171505"
    }
  ]
}

These objects are BeaconCommitteeSelections, so we need to change the type here to Vec<BeaconCommitteeSelection>.