`.Values.*.*DbUrl`s should be incorporated into `chaingraph-secrets`

[bitjson]

Instead of just dropping the values of .Values.hasura.overrideDbUrl, .Values.hasura.metadataDbUrl, and .Values.postgres.externalDbUrl directly into environment variables, those environment variables should be set from the chaingraph-secrets object like postgresConnectionString. This ensures that other systems never unintentionally leak the connection information. (Secrets don't get automatically logged or displayed in many places, simple environment variables do.)

E.g. see difference in treatment here:

https://github.com/bitauth/chaingraph/blob/ee3c284b84e99923906dcd848ed8f5a3838b2934/charts/chaingraph/templates/hasura.yaml#L23-L57

And the chart documentation will also need to be updated to better explain how to add/modify those values.