feat: implement gRPC forwarding for workflow adapter

- Add HTTP/2 transport support for gRPC calls
- Implement dispatchGRPC to forward requests to Temporal gRPC server
- Replace 501 Not Implemented with actual gRPC proxy
- Use golang.org/x/net/http2 for HTTP/2 protocol support
- Supports ListWorkflowExecutions and other gRPC methods

internal/serviceadapter/router.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	"golang.org/x/net/http2"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials/insecure"
 
@@ -218,9 +219,33 @@ func (d *Dispatcher) dispatchGRPC(w http.ResponseWriter, r *http.Request, upstre
 	}
 	defer conn.Close()
 
-	d.writeError(w, problem.NewProblem(http.StatusNotImplemented,
-		"about:blank#not-implemented", "Not Implemented",
-		"gRPC forwarding not yet implemented"))
+	// Create HTTP/2 reverse proxy for gRPC
+	// gRPC uses HTTP/2 protocol, so we need an HTTP/2-capable transport
+	upstreamURLObj := &url.URL{
+		Scheme: "http",
+		Host:   host,
+	}
+
+	proxy := httputil.NewSingleHostReverseProxy(upstreamURLObj)
+	proxy.Director = func(req *http.Request) {
+		req.URL.Scheme = "http"
+		req.URL.Host = host
+		req.URL.Path = method.UpstreamPath
+		req.RequestURI = ""
+		req.Host = host
+	}
+
+	// Create HTTP/2 client transport for gRPC calls
+	// gRPC requires HTTP/2 for proper message framing
+	h2transport := &http2.Transport{
+		AllowHTTP: true,
+	}
+
+	// Set the transport on the proxy
+	proxy.Transport = h2transport
+
+	// Serve the request through the proxy
+	proxy.ServeHTTP(w, r)
 }
 
 func (d *Dispatcher) writeError(w http.ResponseWriter, p *problem.Problem) {