Matches https://github.com/ilyagr/pijul-clone as of now, might become out of sync

Migrate `pijul::identity::Complete::prove` to `pijul::remote::prove`

Removes a circular dependency between pijul::identity and pijul::remote, and is the final hurdle preventing them being spun off into their own crates.

Created by  finchie  on August 17, 2023
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