Allow non-fetcheable remote files

[MrHedmad]

This PR adds the ability for Kerblam! to allow non-fetcheable remote files.
This is done by specifying "_" as the URL, so that Kerblam! will skip the GET request, but still treat the local file as remote for all other purposes.
This required some restructuring of how RemoteFile works under the hood, and how kerblam.toml is parsed.

TODO

Before merging, tick all of these boxes:

• cargo test -- --include-ignored passes without errors or warnings.
• The CHANGELOG.md has been updated to reflect these changes.
• Documentation is updated that reflect these changes, or this PR changes nothing that is reflected in the docs.
• @all-contributors is made aware of this PR, or I am already in the all contributors list.