Pass metadata into get_configuration and subscribe_configuration

[blackwood-ra]

Currently, there is no metadata argument for the client methods get_configuration and subscribe_configuration. Passing metadata is required when subscribing to config store changes with PostgreSQL.

To reproduce, use the following configstore.yaml component with a configured local instance of PostgreSQL running:

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: configstore
spec:
  type: configuration.postgresql
  version: v1
  metadata:
    - name: connectionString
      value: host=localhost user=postgres password=[pg_password] port=5432 connect_timeout=10 database=[database_name]
    - name: table
      value: "config_table"

Without including metadata in the subscribe_configuration, an "Invalid Argument" Err is returned with the following message:

failed to subscribe [key] from Configuration store configstore: unable to subscribe to 'config_table'. pgNotifyChannel attribute cannot be empty

[mikeee]

Closed by #118