internal/mcp: pointerize all the things

Use pointers for the protocol's struct types, to future-proof against
them getting large.

Change-Id: I55b204080a41c8140539697ba264e502e912e613
Reviewed-on: https://go-review.googlesource.com/c/tools/+/672575
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/cmd_test.go

@@ -57,7 +57,7 @@ func TestCmdTransport(t *testing.T) {
 		log.Fatal(err)
 	}
 	want := &mcp.CallToolResult{
-		Content: []mcp.Content{{Type: "text", Text: "Hi user"}},
+		Content: []*mcp.Content{{Type: "text", Text: "Hi user"}},
 	}
 	if diff := cmp.Diff(want, got); diff != "" {
 		t.Errorf("greet returned unexpected content (-want +got):\n%s", diff)