[TASK] Run checkProperties on Config Load

[DougMidgley]

Currently config is checked at various parts of execution. As config is loaded once, this can be done at load time once to simplify checks on config validity. Auth config can equally be checked at this point for validity

check fields are all provided and correct "client_id": "", "client_secret": "", "auth_url": "https://.auth.marketingcloudapis.com/", "account_id":

check matching is there for all credentials (auth = config)