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.