Add use_generic_streaming_requests option to make testing server impls easier

[yotamofek]

Motivation

Currently, testing the server trait implementations of client-streaming methods (i.e. requests that receive a stream) is quite difficult, and requires jumping through hoops in order to construct a Streaming object (see #462 for one way of doing it).

Solution

Adds a codegen option that makes the server trait go from this:

trait Test {
    async fn test_request(
        &self,
        req: Request<Streaming<Message>>,
    ) -> Result<Response<Message>, Status>;
}

to this:

trait Test {
    async fn test_request(
        &self,
        req: tonic::Request<
            impl tokio_stream::Stream<Item = Result<Message, Status>> + Send + Unpin,
        >,
    ) -> Result<Response<Message>, Status>;
}

Since Request<Streaming<Message>> impls IntoStreamingRequest<Message = Result<Message, Status>>, the grpc server can continue invoking these handlers in the same way, but it also allows any object implementing Stream<Item = Result<Message, Status>> to be used.
This makes testing much easier.

[yotamofek]

Just realized this uses associated type bounds which were only stabilized in 1.79.0.
How often is the MSRV raised?
The Unpin bound in IntoStreamingRequest<Stream: Unpin> isn't strictly necessary (and might actually make the trait less flexible), and can be either removed or emulated with a named generic param (i.e. test_request<S: Unpin>(req: impl IntoStreamIterator<Stream = S>) or so)

NVM

[tottoto]

I'm not sure if the benefits of adding this feature are worth the increased complexity that comes with adding more features. However, the refactoring of this part that was done to add this feature seems to improve the readability and maintainability of the code, so if it be separated into a separate pull request, I think we can proceed with the discussion separately.

[yotamofek]

Sure, the refactoring is in a separate commit so I'll put it in its own PR. Thanks!

[yotamofek]

#2143

[jovanpavl-db]

Thanks for working on this; currently, it is incredibly painful!