add a command-line command that displays the schema version for ucm

[aryairani]

Is your feature request related to a problem? Please describe.
helpful for caching codebases

If we're doing tests in CI that require downloading some stuff from share, we want to be able to cache it between CI runs. However, different branches of unison may require different codebase schema versions for their CI runs, so the codebase can't necessarily be shared among all branches.

So my thinking was that we could use the codebase schema version as part of the cache key?

Describe the solution you'd like
maybe a unison --schema-version command

Describe alternatives you've considered
One alternative would be to just include the schema version in the unison --version output, but I'm not sure if anyone is depending on the current format of that output. To be useful for the CI use case above, I'd have to be able to parse out the schema version from the unison --version output.

Additional context
Add any other context or screenshots about the feature request here.

[aryairani]

Open to suggestions, but otherwise let's just add the --schema-version command and call it a day.