[Helm Chart] load DB password from third-party secret

[elyzov]

Is your feature request related to a problem? Please describe.
I use zalando operator to deploy postgresql DB nearby. Zalando operator puts postgresql password to separate secret.
But at the moment, there is no ability to load environment variables from third-party secrets using valueFrom syntax.

Describe the solution you'd like
I want to introduce one more extraEnvRaw variable, which will be injected into the env section of container as is:

extraEnvRaw:
  - name: DB_PASS
    valueFrom:
      secretKeyRef:
        name: superset.superset-postgres.credentials.postgresql.acid.zalan.do
        key: password

Describe alternatives you've considered
We can consider one more abstraction over valueFrom syntax. But I prefer to implement more general and flexible solution.

Additional context
I saw such approach has been used in other charts, like fluentd chart.