Support for merging server routers

[kiiadi]

I have a use-case where I have two smithy services that I want to be satisfied by a single server implementation on a single address binding. Currently there is no way to bind multiple Router implementations to the same hyper address binding.

Something like this would be great:

let app: Router = service_a::OperationRegistryBuilder()...

let serviceB: Router = service_b::OperationRegistryBuilder()...

app.merge(serviceB);

[crisidev]

We could do something similar to what Axum is doing here: https://github.com/tokio-rs/axum/blob/main/axum/src/routing/mod.rs#L257

Doc about Axum router merging: https://github.com/tokio-rs/axum/blob/main/axum/src/docs/routing/merge.md

[david-perez]

Closing in favor of #2979.