Proto: add new companion key in storage (Part 1/2)

What

https://docs.google.com/document/d/1ZTi7-7CcE10tO9O307mAI057iirdwAxzvfHuQcVOHAY/edit?tab=t.0#heading=h.oeumxcgkpsfk

https://docs.google.com/document/d/1Xsl2UYq55CvPPncvslVDCRvT8SxL0yEwCbtR5DF6RtM/edit?tab=t.0#heading=h.oeumxcgkpsfk

We need a second key, similar to consensus keys. This MR adds the required storages. There is also a type change and a stitching. Fun!

The second part will update/add RPCs, and add some tests (regression + stitching)

Why

This will be used for attestations

How

We copy/adapt most of the code for consensus keys. We reuse the global table of consensus keys (as explained in the design document), but the other storages are new.

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