What type of PR is this?

Feature

What does this PR do? Why is it needed?

This PR adds those endpoints:

• /prysm/v1/beacon/states/{state_id}/query
• /prysm/v1/beacon/blocks/{block_id}/query
  which follows the endpoint specification.

I added three SSZ containers for response:

class SSZQueryProof(Container):
    leaf: Bytes32
    gindex: uint64
    proofs: List[Bytes32, SSZ_QUERY_MAX_TREE_DEPTH]

class SSZQueryResponse(Container):
    root: Bytes32
    result: ByteList[SSZ_QUERY_MAX_RESULT_LENGTH]
    
class SSZQueryResponseWithProof(Container):
    root: Bytes32
    result: ByteList[SSZ_QUERY_MAX_RESULT_LENGTH]
    proof: Proof

Which issues(s) does this PR fix?

Part of #15587: Expose SSZ-QL as a new endpoint.

Other notes for review

• After discussion with @rkapka, we decided to 1) return in SSZ encoded bytes and 2) only support a single query at this moment.
• The existing containers for testing (e.g., VariableTestContainer) were moved to a new package (github.com/OffchainLabs/prysm/v6/proto/ssz_query/testing) that only has test-only containers. (which results in big LOC 😅)
• sszInfo is modifed to be public, as it is also used in RPC package.
• include_proof is not yet implemented, so the flag is ignored and always return the response without proof.
• Spec document must be helpful to review this PR. Examples will give some real-world examples.

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.