refactor: Add fs::PathToString, fs::PathFromString, u8string, u8path functions

Summary:
There is no change in behavior. This just helps prepare for the
transition from the boost::filesystem to the std::filesystem path
implementation.

Co-authored-by: Kiminuo <kiminuo@protonmail.com>

This is a backport of core#22937 [1/2]
https://github.com/bitcoin/bitcoin/pull/22937/commits/b39a477ec69a51b2016d3a8c70c0c77670f87f2b

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10784