feat: publication of contract update proposal acceptances

[mrnaveira]

Description

• New gRPC method in the validator node for publishing a proposal acceptance:
  • The arguments are the contract_id and the proposal_id
  • The validator node internally uses its own public key for building the acceptance.
  • The signature of the acceptance only has a mock value for now. In the future we probably would want it to be the signature of a hash of a contract update proposal.
  • Calls the new gRPC method in the wallet to publish the transaction
• New gRPC method in the wallet to submit a proposal acceptance. It builds and sends to the network a transaction with the corresponding type and features
• For now, we do not check if the proposal_id is valid, or any other base layer validation.

Motivation and Context

Validator nodes need to be able to publish contract update proposal acceptances, for contracts in which they are included as committee members and an update proposal is issued.

This PR provides a gRPC method in the validator node (and a corresponding one in the wallet) to publish proposal acceptances.

How Has This Been Tested?

New integration test that publishes and mines a proposal acceptance through the new validator node gRPC endpoint