Extract the PrefilledProof::Formatter so it can be reused for compact blocks

Summary:
This diff only moves the logic of this formatter to serialize.h and specialize it for the PrefilledProof struct. There is no change in behavior.

Depends on D11567 and D11453.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11568