tests: new roachtest for job backward compatibility in new schema changer

[Xiang-Gu]

Backport 2/2 commits from #86950.

/cc @cockroachdb/release

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Commit 1: Moved a few functions so that we can reuse them in the next commit. Commit 2: We added a roachtest that tests job backward compatibility in the declarartive schema changer in a mixed-version cluster. Namely, we test that DDL stmts supported in v22.1 can be planed in nodes running in v22.2 and the jobs created can be adopted and finished by nodes running in v22.1.

Fixes: #79840

Release justification: test only changes.

Release note: None

[cockroach-teamcity]

This change is