Upstream submission for separation of Database credentials

Summary

See #62 (closed) for context. Discussions around support for these behaviors resulted in agreement for it to be submitted to the upstream repository before making Big Bang specific modifications.

Will consider this issue complete when an PR is submitted to the upstream project

This will then allow #62 (closed) to be pulled into a sprint after a decision to merge or not merge into the upstream has been made.

If the upstream does not accept the changes, we can roll the submitted changes into the big bang package as modifications.

Expected Behavior

Enabling the use of separate database user accounts for the engine/UI.

Currently UI is explicitly set to use the same credentials as engine (for accessing the same database. This behavior can be modified via existingSecrets, but we could support passing in a configuration for an alternative user.