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/sse_test.go

@@ -48,7 +48,7 @@ func TestSSEServer(t *testing.T) {
 				t.Fatal(err)
 			}
 			wantHi := &CallToolResult{
-				Content: []Content{{Type: "text", Text: "hi user"}},
+				Content: []*Content{{Type: "text", Text: "hi user"}},
 			}
 			if diff := cmp.Diff(wantHi, gotHi); diff != "" {
 				t.Errorf("tools/call 'greet' mismatch (-want +got):\n%s", diff)