retrieve schema version

[ahwagner]

Currently we can use VRS Python to retrieve the software version, but not the version of the VRS schema being used. We should add in a method to determine this.

[github-actions]

This issue was marked stale due to inactivity.

[github-actions]

This issue was marked stale due to inactivity.

[jsstevenson]

I frequently find myself wishing this were available. I'm not sure there's a better solution than declaring a constant somewhere in the source code, though. The relative versionlessness of VRS 2 is also a complicating factor.

[korikuzma]

@jsstevenson added this in #543