Add AutoFile::detail_fread member function

Summary:
New code can call the method without having first to retrieve the raw
FILE* pointer via Get().

Also, move implementation to the cpp file. Can be reviewed with:
--color-moved=dimmed-zebra --color-moved-ws=ignore-all-space

This is a partial backport of core#28060
https://github.com/bitcoin/bitcoin/pull/28060/commits/000019e158ef01f2bedc3fc1589f95e106e817ea
Depends on D18982

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18983