Router: add_service / merge for top-down multi-service registration

[iainmcgin]

Registering multiple services with a Router currently reads inside-out:

let router = Arc::new(svc_b).register(Arc::new(svc_a).register(Router::new()));

or, more readably but with intermediate bindings:

let mut router = Router::new();
router = Arc::new(svc_a).register(router);
router = Arc::new(svc_b).register(router);

The register extension trait on Arc<Self> is a deliberate design — it avoids the visitor-pattern wrapper that tonic generates (GreeterServer::new(impl) / GreeterServer::from_arc(impl)) — but the call shape inverts the data flow compared to every other framework in the ecosystem:

// tonic
Server::builder()
    .add_service(GreeterServer::new(greeter))
    .add_service(EchoServer::new(echo))
    .serve(addr).await?;

// connect-go (each service mounts independently on the mux)
mux.Handle(elizav1connect.NewElizaServiceHandler(eliza))
mux.Handle(greetv1connect.NewGreetServiceHandler(greeter))

A user arriving from tonic types Router::new().add_service(...) first, gets a method-not-found, and has to read the codegen output to discover the register extension trait.

Proposed direction

Add a forwarding builder method on Router so registration reads top-down:

impl Router {
    /// Register a service. Equivalent to `Arc::new(svc).register(self)`.
    pub fn add_service<S, E>(self, svc: Arc<S>) -> Self
    where
        S: ?Sized,
        Arc<S>: ServiceRegister,   // a small trait the codegen `*Ext` traits could implement
    {
        svc.register(self)
    }
}

so the call site is:

let router = Router::new()
    .add_service(Arc::new(eliza))
    .add_service(Arc::new(greeter));

This is purely additive — register(self: Arc<Self>, router) stays as the underlying mechanism — but it gives the discoverable spelling. While here, also consider promoting the merge_routers free function to an inherent Router::merge(self, other) -> Self (and an in-place Router::extend(&mut self, other) sibling) for the same discoverability reason.

Scope

Small: one pub trait for the bound, one inherent method, doc update, example update.