Add function to checkout the jobs repository

[ColinKinloch]

This branch adds crate::clone_git_repository to clone git repos generally and Job::clone_git_repository to clone specifically defined by the gitlab job.

clone_git_repository is heavily based on PrepareFetch::fetch_only

Since the gitoxide PrepareFetch and PrepareCheckout structs delete the target path when they are dropped I opted to checkout the repository to a temporary directory, then move it's contents to the path provided to the target directory.

This adds a rather unwieldy GitCheckoutError enum. gitoxides Error types triggers the result_large_err clippy warning.

This requires rust version 1.73.0 or later.

Fixes #21

[andrewshadura]

This looks okay, but I haven’t gone deeply into details. I trust you tested this code well? (It would be great if it also included some automated tests.)