CircleCI: Add separate approval step for oa utests on separate workflows

CASSANDRA-14227 added new j*_utests_oa jobs to run unit tests with the new, not-default "oa" sstable format. Those tests depend on the start_j*_unit_tests approval step on the separate workflows.

However, the separate workflow is meant to allow running jobs individually, and having a common approval step for both j*_unit_tests and j*_utests_oa prevents us from doing so. That's the case for example of the development of CASSANDRA-18504, where I want to run a single unit test job without caring about the specialization, but the common approval step forces me to run both jobs, duplicating the costs. An example run can be seen here.

I think that j*_utests_oa should have its own separate approval step, the same way that j*_utest_cdc, j*_utest_fqltool, j*_utest_compression or j*_utest_system_keyspace_directory have their own approval step.