internal/mcp: implement cancellation

Use the existing jsonrpc2 preemption mechanism to implement MCP
cancellation.

This was mostly straightforward, except where I got confused about ID
unmarshaling. Leave a note about using omitempty when it is available.

Change-Id: I13e073f08c5d5c2cc78d882da4e6ff47f09fb340
Reviewed-on: https://go-review.googlesource.com/c/tools/+/667578
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/protocol/generate.go

@@ -56,6 +56,9 @@ var declarations = config{
 	"CallToolResult": {
 		Name: "CallToolResult",
 	},
+	"CancelledNotification": {
+		Fields: config{"Params": {Name: "CancelledParams"}},
+	},
 	"ClientCapabilities": {Name: "ClientCapabilities"},
 	"Implementation":     {Name: "Implementation"},
 	"InitializeRequest": {
@@ -73,7 +76,8 @@ var declarations = config{
 	"ListToolsResult": {
 		Name: "ListToolsResult",
 	},
-	"Role": {Name: "Role"},
+	"RequestId": {Substitute: "any"}, // null|number|string
+	"Role":      {Name: "Role"},
 	"ServerCapabilities": {
 		Name: "ServerCapabilities",
 		Fields: config{