Proto: add update companion key operation

What

Follows !17236 (merged)

Add the operational side of companion key update, with validation, application, and client handling.

Tests will be added in a subsequent MR.

Why

Continuation of !17213 (merged), this MR adds the protocol side code that allows us to fill this storage. This MR unlocks the implementation of integration tests for this feature.

How

The operation representation of Update_consensus_key is extended with an additional field, instead of creating another operation. The rest follows.

Manually testing the MR

One should be able to register a companion key in a test environment (i.e sandbox mode) and see (with RPCs) that the key is being registered adequately.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR