WIP: url with trailing slash mirror

[wolfv]

@baszalmstra I refactored things to use the URLWithSlash in the mirror.

Unfortunately, that type currently lives in rattler_conda_types. I need it in rattler_networking though. They both currently don't have a relationship.

We could add another utils crate or make one dependent on the other. Not sure. Happy about suggestions.

This PR builds on top of #939 and I'd suggest merging that one first.

[baszalmstra]

I suppose adding another crate lije url-utils or whatever would help? We could also rename file_url and add it there?

[wolfv]

Do you think file-url and urlwithslash could be in a UrlExt crate or something? Or something rattler-specific.