Select for properties of targets such as version in pants queries/filters

[dannytodd]

Describe the solution you'd like
I would like to be able to query, via pants, properties of targets rather than simply the targets (and their transitive dependencies) themselves. Namely, the version parameter of all targets that support it. This would enable triggering of release builds of packages when a version bump occurs by querying with something akin to:

pants list --filter-target-type=python_distribution --changed-property=version

This is a common pattern when dealing with file-level changes in other systems, such as triggering on changes to a __version__ file. The added granularity of being able to handle target properties directly would be useful in lots of cases, but triggering on version updates would be a good first example.

Describe alternatives you've considered
The basic alternative would be to grep the git diff of BUILD files, but this isn't very robust.