(feat request) helm chart should support building compound connection vars

[dominicdejacomo]

An increasingly common cloud provider configuration for databases is to require SSL for the DB connection, this requires using the MB_DB_CONNECTION_URI env var to set extra parameters.

Additionally, some cloud providers format their username user@instance which causes the upstream URI env var parser to break as it's not expecting @'s in the username field.

Per metabase/metabase#22862, when you need to set advanced MB_DB_CONNECTION_URI parameters and your username contains an @, you should pass MB_DB_CONNECTION_URI, MB_DB_USER and MB_DB_PASS together.

This helm chart currently uses an if/else workflow for mapping those three fields.

I've prepared and locally tested a pull request which has more granular control of rendering those variables to support this use case.