internal/mcp: support prompts

Add support for 'prompts/list' and 'prompts/get', and registering
prompts on the server using AddPrompts. Add a 'MakePrompt' helper to
construct prompts from a prompt handler using reflection.

Change-Id: I479cb9c9b99313cca99640c1f5ea1939363759e6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

mcp/sse_example_test.go

@@ -40,7 +40,7 @@ func ExampleSSEHandler() {
 	}
 	defer serverConn.Close()
 
-	content, err := serverConn.CallTool(ctx, "add", AddParams{1, 2})
+	content, err := serverConn.CallTool(ctx, "add", map[string]any{"x": 1, "y": 2})
 	if err != nil {
 		log.Fatal(err)
 	}