Refactor signatures into CommitSignature module

What does this MR do and why?

Describe in detail what your merge request does and why.

Commit signatures are currently top-level models and have some duplicated code. This change moves the signatures into a CommitSignature module, and moves shared code into a concern.

This will also facilitate the addition of new signature methods, like SSH signatures.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

N/A: Non-visible refactor

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

N/A: Non-visible refactor

MR acceptance checklist

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