feat(examples): add grpc-web example (#710)

Creates a server using tonic-web and a client that uses a regular
`hyper::Client` and issues a regular HTTP/1.1 request.

examples/src/grpc-web/server.rs

@@ -0,0 +1,49 @@
+use tonic::{transport::Server, Request, Response, Status};
+
+use hello_world::greeter_server::{Greeter, GreeterServer};
+use hello_world::{HelloReply, HelloRequest};
+
+pub mod hello_world {
+    tonic::include_proto!("helloworld");
+}
+
+#[derive(Default)]
+pub struct MyGreeter {}
+
+#[tonic::async_trait]
+impl Greeter for MyGreeter {
+    async fn say_hello(
+        &self,
+        request: Request<HelloRequest>,
+    ) -> Result<Response<HelloReply>, Status> {
+        println!("Got a request from {:?}", request.remote_addr());
+
+        let reply = hello_world::HelloReply {
+            message: format!("Hello {}!", request.into_inner().name),
+        };
+        Ok(Response::new(reply))
+    }
+}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tracing_subscriber::fmt::init();
+
+    let addr = "127.0.0.1:3000".parse().unwrap();
+
+    let greeter = MyGreeter::default();
+    let greeter = GreeterServer::new(greeter);
+    let greeter = tonic_web::config()
+        .allow_origins(vec!["127.0.0.1"])
+        .enable(greeter);
+
+    println!("GreeterServer listening on {}", addr);
+
+    Server::builder()
+        .accept_http1(true)
+        .add_service(greeter)
+        .serve(addr)
+        .await?;
+
+    Ok(())
+}