Proto: add new companion key in storage (Part 2/2: RPC and tests)

What

Follow-up of !17213 (merged)

Why

How

Manually testing the MR

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

Summary by CodeRabbit

• New Features

  • Introduced a new API endpoint to retrieve a delegate’s active companion key (returns “none” if unset).
  • Enhanced the delegate information API to include companion key details along with pending key updates.
  • Expanded companion key management to support dynamic key updates and improved cryptographic handling.

• Documentation

  • Updated protocol documentation to reflect the new and enhanced companion key features.