Draft: Add a service to validate Demonstrate Proof of Posession headers

⚠ NOT FOR MERGING! Keep in draft ⚠

This is so Nick can clarify some questions with the original MR's author.

What does this MR do and why?

Add a service to validate Demonstrate Proof of Posession headers

This MR takes code from @ameyadarshan's original MR at !148175 (closed)

It focusses just on adding the service needed to validate the DPoP header, validate it, find the referenced user, and validate the DPoP signature (made using the user's public key) against the user's private key.

Work in progress.

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Related to #425130