Enable ability for DBURI to be an existing secret

[justmitch123]

Hey,

This is hopefully a nice and simple PR. This resolves https://github.com/plankanban/planka/issues/750 by adding an existingDburlSecret field to replace the dburl if provided to securely pass the connection string into the environment.

I am not quite sure if i am happy with the naming of the variables etc in the top level but it was the closest I could come up with to match similar https://github.com/plankanban/planka/blob/master/charts/planka/values.yaml#L154 values.

I feel like it might work nicer to nest this parameter within the postgres object for consistency but either option works for my use case.

I have a helm playground here that tests out the proposed changes

Let me know if you have any question or you would like me to make any other modifications to the PR.

If these changes are approved & successful, I am keen to start moving some of the other env variables out to secrets - i.e DEFAULT_ADMIN_PASSWORD & SECRET_KEY. The main use case for this is that we will have developers that have access to view pod definitions etc, but we restrict decryption of secrets

[CLAassistant]

All committers have signed the CLA.

[meltyshev]

Thanks for this addition 🙏