feat: Add sender validation to block proposal RPC

[jbencin]

Description

Require block proposals to be signed by an approved signer before proceeding to parse and validate the proposal. Because validating a block proposal is an expensive operation, allowing anyone to submit a proposal would be a potential Denial of Service attack vector.

See corresponding issue for full details.

Applicable issues

• fixes Add sender validation to block proposal RPC #135

Additional info (benefits, drawbacks, caveats)

• I know I'm going to need to add some integration tests. I'm creating the pull request now in order to get some feedback on the code and some advice on testing
• I chose to hex encode the proposal message before signing it. This allows us to validate the message signature before fully deserializing the JSON. I chose hex encoding over base64 to avoid introducing an extra dependency
• Membership in the set of allowed signers is currently checked using Vec::contains(). Depending on the size of the signer set, it may be good to change this to a HashSet (or BTreeSet), but that would require a code change in stacks-blockchain: Deriving Hash (or Ord) for Secp256k1PublicKey

Checklist

• Test coverage for new or modified code paths
• Changelog is updated
• Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo
• New integration test(s) added to bitcoin-tests.yml

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #211 (54cb37b) into master (54c653c) will not change coverage.
The diff coverage is n/a.

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[kantai]

This looks great to me! My only comment is on the message field -- it seems like it would make more sense for that to be a Proposal type rather than the String, but I could be convinced otherwise.

[obycode]

lgtm!