fs: consistently use fsbridge:: for ifstream / ofstream

Summary:

fs: consistently use fsbridge for {i,o}fstream

fs: add missing <cassert> include

This is needed to prevent compilation failures once boost is removed,
however is still correct to include now, and reduces the diff in #20744.

<string> is extracted from the defines because it is used for Windows
and non-Windows code, i.e get_filesystem_error_message().

fs: consistently use fsbridge for fopen()

This is a backport of core#24167

Depends on D11984

Notes:

• the psbtoperationsdialog.cpp and fuzz.cpp changes are not applicable due to missing backports
• the walletframe.cpp changes are in walletview.cpp because of missing backports (core-gui#399)

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari, Fabien

Reviewed By: #bitcoin_abc, sdulfari, Fabien

Subscribers: Fabien, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11975