Helm chart: Add "extraEnvVars"

[jonerer]

This change allows users to add additional env vars to the helm release.

We needed this in order to be able to populate our kafka properties in a secure way. For instance with this we use:

extraEnvVars:
  - name: "KAFKA_PROPERTIES"
    valueFrom:
      secretKeyRef:
        name: kafdrop-kafka-properties
        key: properties

This change should be backwards compatible.