Add etags to acm_service_perimeter_resource

[modular-magician]

Important

This PR is dependent on #12735. I'd appreciate it if we can try to get both of these in the end of Tuesday cut off for next weeks release. Thanks!

The acm_service_perimeter_resource is an item in a list (like a project in a list of projects). Since there is no API for "add item to list", terraform actually makes a GET then PATCH to get the current list, merge them, then send the combined list back to the API.

This GET-PATCH pattern can override data when concurrent requests are sent. So, we're trying to add etags to block requests that can result in data loss. Previously, I added the new post_read custom code hook so we can retrieve the etag.

I've tested this locally and I can see the etags being sent in the PATCH requests.

Please let me know if you have any questions.

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accesscontextmanager: added `etag` to `google_access_context_manager_service_perimeter_resource` to prevent overriding list of resources

Derived from GoogleCloudPlatform/magic-modules#12736