Add files.copy operation

[EricDriussi]

Adds copy operation suggested in #1121. Any and all feedback is welcome!

• [x] Pull request is based on the default branch (3.x at this time)
• [x] Pull request includes tests for any new/updated operations/facts
• [x] Pull request includes documentation for any new/updated operations/facts
• [x] Tests pass (see scripts/dev-test.sh)
• [x] Type checking & code style passes (see scripts/dev-lint.sh)

[EricDriussi]

Hey! Sorry it took me forever to get back to this... I implemented the changes mentioned here, added/fixed tests and skipped the Windows CI (simply due to me not having a Windows machine to ensure the tests work before pushing).

Also, in the new _remote_file_equals function, I followed the same fallback logic I saw in _file_equals regarding the checksum comparison. I'm not quite sure if this is correct so please let me know if there's anything that needs to be updated before merging.