Add custom registry support

[fujiapple852]

@obi1kenobi this PR is a rebase (and rework) of @drewkett's earlier PR https://github.com/obi1kenobi/cargo-semver-checks/pull/945, to add support for custom cargo registries, on top of the latest main. I say rework as main has changed quite a bit since the original PR and so a simple rebase was not enough.

This PR also adds an integration test for the custom registry functionality. The test use a static file mango registry which is served via an ephemeral axus instance. The test is self contained and does not require any external resources.

Finally, an unrelated bug I observed, with the regex used for the bugreport test, is resolved. This could be pulled out into a separate PR if needed.

The goal here is to get this into a suitable state such that it could be merged to main to thus avoid the need to maintain a downstream fork. I have kept my changes separate from @drewkett's original changes to aid in reviewing the delta, but the intention is to eventually squash these.

Thanks!