roachtest: add mixed version testing for the schema changer corpuses

[fqazi]

Previously, we had no automated tests for validating mixed version compatibility. This new roachtest validates elements generated on the previous release against the current. It validates elements generated in a mixed version state on the current release against previous version. It also sanity checks that any existing corpus stays compatible with the current version. The corpuses uses are automatically generated on nightly builds and stored on google cloud storage.

Release justification: low risk and enhances backwards / forwards compatibility test coverage for declarative schema changer. Release note: None

[cockroach-teamcity]

This change is 

[fqazi]

RFAL - Infrastructure is in place for this now

[fqazi]

bors r+

[craig[bot]]

Build succeeded:

• Bazel Essential CI (Cockroach)