Make the tonic-web layer axum compatible

[MarkusTieger]

Motivation

So tonic-web can be used with axum-multiplexing.
For example: (GenericAuthenticationServiceServer and GenericAuthenticationService are from my own project, don't get confused)

let mut routes = Routes::builder();

routes.add_service(GenericAuthenticationServiceServer::new(
   crate::services::auth::GenericAuthenticationService {
     backend: self.backend.clone(),
   },
));
let router: axum::Router = routes
            .routes()
            .into_axum_router()
            .layer(GrpcWebLayer::new());

// This router can now be used in combination with normal http routes.

Solution

With this pr you can just .layer(GrpcWebLayer::new()) on a normal axum router (as long as the axum feature is enabled)

Fixes #1964