Split backsyncer into multiple files

Summary:
# This Stack
Migrate backsyncer CLI to use mononoke_app + clap 4, instead of old cmdlib + clap 2.

I might also make some improvements to other cross-repo CLIs, e.g. forward syncer, bookmarks validator.

## This diff

Organizes the code a bit before making changes, by splitting it into multiple files.

Reviewed By: clara-9

Differential Revision: D68957189

fbshipit-source-id: b68d5b2bc2a95cdcdf74ee2558114595048eeb29