feature request: mathesar: be able to use a secret

[bbigras]

for SECRET_KEY and POSTGRES_USER.

Maybe something like what registry-1.docker.io/bitnamicharts keycloak uses:

externalDatabase:
  existingSecret: "postgresql-cluster-app"
  existingSecretHostKey: "host"
  existingSecretPortKey: "port"
  existingSecretUserKey: "user"
  existingSecretDatabaseKey: "dbname"
  existingSecretPasswordKey: "password"