Add NotificationConfig to google_container_cluster

[modular-magician]

This PR aims to introduce the new beta attribute NotificationConfig to google_container_cluster. This is the issue I created in the terraform provider (though in the wrong provider, I should've added it to the beta provider): hashicorp/terraform-provider-google#7337

In order to test this PR I ran:

• bundle exec compiler -a -v "beta" -e terraform -o "$GOPATH/src/github.com/hashicorp/terraform-provider-google-beta" to generate the provider
• make test in the provider
• make lint in the provider
• rake test in magic modules

References:

• https://cloud.google.com/kubernetes-engine/docs/how-to/cluster-upgrade-notifications?hl=el
• https://cloud.google.com/kubernetes-engine/docs/reference/rest/v1beta1/projects.locations.clusters#Cluster.NotificationConfig

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

container: Added `notification_config` to `google_container_cluster`

Derived from GoogleCloudPlatform/magic-modules#4012