Update metabase chart to use existingSecret pattern

[themightychris]

Helm charts these days, instead of having you input secret values as helm values, often use existingSecret keys to pull values out of secrets that are assumed to be put into place out of band.

The metabase values file currently postgresql connection details as values: https://github.com/JarvusInnovations/infra-components/blob/main/charts/metabase/values.yaml#L42:L53

The existingSecret file should be expected to have POSTGRES_USER and POSTGRES_PASSWORD keys and pull from there for both configuring the provisioned database and configuring metabase to connect to it. All the other fields can/should stay values