Use suppressdiff on monitor type

[nmuesch]

This PR takes us back to using the suppressDiff function for the case of monitor types being one of metric alert or query alert

The issue with the previous approach was that the diff is calculated based on what the state is refreshed as vs what the config has. Take this example:

User provides metric alert in the config. Runs terraform apply and the API successfully creates the monitor (but the monitor is really a query alert. Future API calls return query alert in the response.
With older monitors (created before Provider 2.0.0), the state contained query alert because the diffsuppress ignored the diff and set the state to contain what the API returned.
With 2.0.0 and 2.0.1, since this suppression was moved into the Read function, the state wasn't updated with the api response, but contained what the config had. This caused (expected) no diff to be created with new monitors, but any old monitor would be recreated.

This should fix #241

[bkabrda]

LGTM 👍

[gzussa]

A quirk indeed 😄

[nmuesch]

Thanks for the reviews! I've tested this one manually a few times by manually changing my state to have different types (not sure if we can do that in the testing framework) Based off this I'm going to merge.