CMP Stub postMessageEventHandler should prevent attempting to parse unrelated messages

[nealrosen]

Version
1.3.1

Module (core, cmpapi, cli, stub, or testing)
stub

Describe with reproduction steps – What is the expected behavior?
On some sites, there can be a large number of postMessage calls made for various purposes unrelated to the CMP. As a result, the stub attempts to parse a lot of messages that are not JSON formatted and/or are not intended for the CMP. This results in many caught exceptions thrown. While not directly user impacting, this takes up unnecessary CPU cycles and makes debugging with caught exceptions very difficult with the caught exception "noise" coming from the CMP.

This could be mitigated significantly with a couple simple checks against string message payloads before attempting to parse them as JSON. A couple simple checks might be

1. Check that the payload starts with a curly bracket
2. Check that the payload contains "tcfapi"