Extend the local transport URL parser to accept the standard Git <transport>::<address> format (see gitremote-helpers docs [1]) for remote helper composition.

This allows radicle repositories to have other remote helpers operate on the git repo, before content is handed over to git-remote-rad, as per the spec.

Note: Url::from_str accepts the prefixed form but Display always outputs canonical rad:// — the transport prefix is not part of the Radicle URL identity, so the round-trip is lossy by design.

[1] - https://git-scm.com/docs/gitremote-helpers#_invocation

Signed-off-by: Yorgos Saslis yorgos.work@proton.me