Consider exposing an aspect CLI plugin

[alexeagle]

We could add a third entry point next to target-determinator and driver which would contribute a command to Bazel, by implementing a https://docs.aspect.build/aspect-build/aspect-cli/5.0.1/docs/help/topics/plugins.html

If you're up for it, I could get this started. I'd like to be able to run target-determinator just with bazel affected.

[illicitonion]

Very happy for this, particularly to review PRs! IIRC @shs96c may have some prior art to build on here too.

One thing to call out is we explicitly added a BazelCmd interface to make something like an Aspect plugin easier to write, but note that we need the ability to run bazel from a working directory other than the workspace, which IIRC was an API difference.