add support for Configuration API

[dmitsh]

The Configuration API feature has been implemented in Dapr runtime: dapr/dapr#3606
It needs to be accessible via the SDK.

[amulyavarote]

/assign

[chinzhiweiblank]

Good day all, I would like to take this on.

These are the preliminary steps I would take.

1. Generate the proto client
2. Implement GetConfigurationItem, GetConfigurationItems, SubscribeConfigurationItems, UnsubscribeConfigurationItems.
3. Test out locally and then create tests

[wcs1only]

@amulyavarote has already begun implementation on this, but if the two of you would like to collaborate, or trade work I'll leave that up to you.

[chinzhiweiblank]

No problem. I can take on this issue instead.