nixos/cockroachdb: create new service

This also includes a full end-to-end CockroachDB clustering test to
ensure everything basically works. However, this test is not currently
enabled by default, though it can be run manually. See the included
comments in the test for more information.

Closes #51306. Closes #38665.

Co-authored-by: Austin Seipp <aseipp@pobox.com>
Signed-off-by: Austin Seipp <aseipp@pobox.com>