Automatically set postgrescluster.enabled to false if postgresql.type != 'postgrescluster'

[batpad]

Currently, if you set postgresql.type to external-plaintext for example in a values over-ride, you will get an error as it still tries to create the PostgresCluster resource.

Therefore, one also needs to set postgrescluster.enabled to false to work with an externally managed database.

This feels a bit redundant. It would be nice if the postgres cluster setup was automatically disabled if postgresql.type was set to anything other than postgrescluster.

Really not a huge deal, just an ergonomics thing - noticed this while setting up eoapi-k8s for WRI.

cc @emmanuelmathot @pantierra

[pantierra]

postgrescluster is a subchart , while postgresql.type is from the main chart. Helm evaluates dependency conditions statically before template rendering, so we can't dynamically disable the postgrescluster subchart based on postgresql.type.

To address this I propose to add some hints in comments/documentation and we can use some helpers to validate and give an error message when postgresql.type is set to external-plaintext or external-secret but postgrescluster.enabled is still true.

Please check #346 for a concrete implementation.