[FEATURE] add RDS persistence option to `mlflow-fargate` module

[kukushking]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
I would like to be able to use RDS for mlflow metadata storage. mlflow-fargate module should provide configuration options to support RDS instead of default EFS-based persistence. The mlflow-image module should be adapted to allow specify database URI to connect to and store experiments and model metadata.

Describe alternatives you've considered
Alternative is already built-in via EFS volume mounted to mlflow-fargate. This adds another option for RDS-backed persistence.

Additional context
Add any other context or screenshots about the feature request here.

[kukushking]

We should use RDS module from IDF awslabs/idf-modules#125 when the PR is merged (& release is cut).