refactor: Split util::AnyPtr into its own file

Summary:
It makes sense to keep util::insert and util::AnyPtr functions in util/, not common/ as these are header-only template functions with no dependencies, and there's no particular reason they shouldn't be used by kernel code even if they aren't used right now.

This is a partial backport of core#27636
https://github.com/bitcoin/bitcoin/pull/27636/commits/7eee356c0a7fefd70c8de21689efa335f52a69ba

Depends on D16213

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16214