Add third party registry support

[drewkett]

This adds a --registry command line flag to the check release capabilities. The argument to the flag is the name of the registry to use to lookup the crates when using the default behavior or --baseline-version. If left unset, the default behavior remains of --checking crates.io.

This is intended to address #160.

I avoided trying to do any auto detection based on publish fields. That seemed like it might be less obvious from a behavior perspective to me, but maybe it would make things smoother for the user.

[drewkett]

I wasn't sure if this was the best place to stash the registry flag from the cli. It seems like its okay, but I can't say I did a deep dive of the code to fully understand all the components.

[drewkett]

I haven't added any tests for this. I'm not sure what the easiest way to add tests for a third party registry. Do you have any ideas on that front?

I could wire up a test to basically just pass --registry crates.io to validate that the flag works in that case.

As far as third party registries, one thought I had was to setup a static file based registry with margo. That does require at least spinning up a local host based web server, but I think that wouldn't be too much trouble to make work.

Let me know what route you would prefer and some pointers on where exactly the tests should go.

[suaviloquence]

As far as third party registries, one thought I had was to setup a static file based registry with margo. That does require at least spinning up a local host based web server, but I think that wouldn't be too much trouble to make work.

See also #879 for some discussion about using margo for testing.

[fujiapple852]

Rebased and reworked this PR against the latest main and also added an integration test: #1048