Move FopenFn to fsbridge namespace

Summary:
[META] In a future commit in this patchset, it will be used by more than just validation, and it needs to align with fopen anyway.

This is a partial backport of core#25487
https://github.com/bitcoin/bitcoin/pull/25487/commits/b3267258b052557fc136b9a4dcb754afb9219470

Depends on D15719

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15720