Nix/module: add defaultSettings

[fufexan]

Describe your PR, what does it fix/add?

TO BE MERGED AFTER #9930

Add defaultSettings option in the NixOS module. Can be disabled by setting it to {}.

Default settings currently includes allowing screencopy for XDPH and grim, and optionally adding exec-once = uwsm finalize in case withUWSM = true.

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

Brainstorming is welcome. This is what made sense to me at the time of opening this PR.

Is it ready for merging, or does it need work?

Untested, no.