Add Support for GraphQL SUBSCRIPTION in gRPC-GraphQL Wrapper

[d15c0d3r]

The current implementation of the gRPC-GraphQL wrapper lacks support for the GraphQL SUBSCRIPTION type.
Received an error while generating proto files for grpc endpoint with type : SUBSCRIPTION as an option

rpc TestSubscription(TestSubscriptionRequest) returns (TestSubscriptionResponse) {
  option (graphql.schema) = {
    type: SUBSCRIPTION
    name: "test_subscription"
  };
}

proto:106:31: Error while parsing option value for "schema": Unknown enumeration value of "SUBSCRIPTION" for field "type".
make: *** [proto] Error 1)

This limitation prevents real-time updates and event-driven use cases, which are critical for applications requiring live data feeds or asynchronous event handling.