feat: add ability to echo headers & sample routing annotations on echo method

[alicejli]

As part of supporting dynamic routing annotations per google/api/routing.proto, this adds the following:

1. Several sample routing annotations to the Echo.Echo method
2. The ability for each Echo service to echo the contents set with the x-goog-request-params metadata key to the response headers

An example of how to set and read the headers in Go:

func main() {
	conn, err := grpc.Dial("localhost:7469", grpc.WithInsecure())
	// This is what the generated client library should be creating
        md := metadata.New(map[string]string{"x-goog-request-params": "testHeader"})
	if err != nil {
		log.Fatal(err)
	}
	defer conn.Close()
	opt := option.WithGRPCConn(conn)
	ctx := metadata.NewOutgoingContext(context.Background(), md)

	echo, err := showcase.NewEchoClient(ctx, opt)
	if err != nil {
		log.Fatal(err)
	}
	defer echo.Close()

	content := "HELLO WORLD"

	req := &showcasepb.EchoRequest{
		Response: &showcasepb.EchoRequest_Content{
			Content: content,
		},
	}
	// Very important step! Another metadata instance must be provided to the `grpc.Header()` with which to capture them.
	mdForHeaders := metadata.New(map[string]string{})

	resp, err := echo.Echo(ctx, req, gax.WithGRPCOptions(grpc.Header(&mdForHeaders)))

	fmt.Printf("This is the response %v\nThis is the header:%v", resp, mdForHeaders.Get("x-goog-request-params")

	if err != nil {
		log.Fatal(err)
	}
}

The result should be:

This is the response content:"HELLO WORLD"
This is the header:[testHeader]

Each language microgenerator can use the following sample tests to the Echo method to confirm the dynamic routing annotations are being generated properly.

1. EchoRequest without header or other_header field specified
   Header should be empty.

EchoRequest{
   other_header: “waldo”
}

Expected Header: baz=waldo

EchoRequest{
   header: “foo”
}

Expected Header: routing_id=foo&header=foo

EchoRequest{
   header: “projects/foo”
}

Expected Header: routing_id=projects/foo&header=projects/foo

EchoRequest{
   header: “projects/foo/instances/quuz/rows”
}

Expected Header: instance_id=instances/quuz&table_name=projects/foo/instances/quuz/rows&header&super_id=projects/foo&routing_id=projects/foo/instances/quuz/rows&header=projects/foo/instances/quuz/rows

EchoRequest{
   header: “regions/foo/zones/bar/rows””
}

Expected Header: table_name=regions/foo/zones/bar/rows&routing_id=regions/foo/zones/bar/rows&header=regions/foo/zones/bar/rows

EchoRequest{
   header: “projects/foo/instances/bar”
}

Expected Header: super_id=projects/foo&routing_id=projects/foo/instances/bar&header=projects/foo/instances/bar

EchoRequest{
   header: “thud"
   other_header: “projects/foo/instances/bar”
}

Expected Header: quz=projects/foo&baz=projects/foo/instances/bar&routing_id=thud&header=thud

[noahdietz]

Looks great.

[noahdietz]

I'm happy with this. I will let you, @alicejli decide if you want eyes from the other reviewers.