Proposal: Querying state store

[ruokun-niu]

The state query API is an existing API that is currently in the alpha stage. The Rust SDK does not currently support this.

Since the API already exists, we do not need to make any changes to the state query API. I propose that we can add a new function called query_state_alpha1() inside the Client struct, which is consistent with the other SDKs. It will use the existing proto message QueryStateRequest and QueryStateResponse.

Given that this API is in still in Alpha, I'm not sure if it makes sense to add this to the main crate or to create some kind of experimental/preview that contains all of the APIs that are in alpha stage.