Expose extensions map in Request

[ebkalderon]

The underlying http::Request type from which the tower_grpc::Request type is made from has a useful method called extensions_mut(), which can store arbitrary data alongside the request, as long as it is Send + Sync + 'static. The data can then be queried later with:

if let Some(extra_metadata) = request.extensions().get::<MyMetadata>() {
    // Respond to request one way.
} else {
    // Respond to request another way.
}

It would be nice to preserve this metadata in the tower_grpc::Request and make it accessible through an extensions() method.

[per-gron]

I explained some concerns I have about exposing HTTP and language interoperability in #95, but I still like the idea of having a getter that returns an Option like this, especially if the API would be easy to use with protobuf types. I think an API that looks almost the same as this could be built on top of gRPC custom metadata primitives (which are being worked on now in #92 and #90).

[ebkalderon]

Sounds good. I've decided to close the associated PR #95 as well as this issue in favor of waiting for your upcoming PR with MetadataMap support to land. Thanks for working on this.