Resolve "Rollout multi-DB CI testing for gitlab_sec"

What does this MR do and why?

This MR configures CI tests that execute GitLab tests on a configuration using a dedicated sec database as part of the sec-decomposition. This assists us to expose cross interaction issues that are not visible when GitLab's code is not actively interacting with a separate database connection.

These tests are configured only to run on MR's with the sec-decomposition label on it, and only when the SEC_DECOMPOSED_DB env variable is set to true. This is not done in this MR so that we can successfully merge the configuration. It is however, enabled in this MR to begin to expose the cross connection interaction issues, which additionally proves that this configuration works.

Related to #493983 (closed)