Add user labels and labels to monitoring notification channel DS

[emilymye]

Also fix a test --> Fixes hashicorp/terraform-provider-google#5452

The data source fails if multiple channels of a type exist, so a test that only tests type will fail if any dangling resources exist with the same type, or any tests with a same type are running concurrently. Thus, we have two type tests that test with labels to try to cover this, so we don't run into test errors. I think the real solution is that we should add/replace this with a data source allowing for listing multiple channels.

Also changed some urls/emails to be safer-ish.

Release Note Template for Downstream PRs (will be copied)

monitoring: Added `labels` and `user_labels` filters to data source `google_monitoring_notification_channel`

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff
Terraform Beta: Diff

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 3079a9e.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#1666
depends: hashicorp/terraform-provider-google#5470

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff
Terraform Beta: Diff

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 651bcae.

Pull request statuses

terraform-provider-google-beta already has an open PR.
No diff detected in terraform-google-conversion.
terraform-provider-google already has an open PR.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I didn't open any new pull requests because of this PR.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff
Terraform Beta: Diff