Add waiting room event

[broswen]

Add support for managing Waiting Room Events in Terraform.

cloudflare_waiting_room_event depends on a cloudflare_waiting_room to be created. Added into the Terraform template to create the acceptance test resources.

This is my first Terraform contribution so please let me know if there are things that should be done differently.

[github-actions]

This project handles dependency version bumps (including upstream changes from cloudflare-go) independently of the standard PR process using automation. This allows the dependency upgrades to land without causing merge conflicts in multiple branches and handled in a consistent way. The exception to this is security related dependency upgrades but they should be co-ordinated with the maintainer team privately.

Please remove the changes to the go.mod or go.sum files from this PR in order to proceed with review and merging.

[github-actions]

Oops! It looks like no changelog entry is attached to this PR. Please include a release note as described in https://github.com/cloudflare/terraform-provider-cloudflare/blob/master/docs/changelog-process.md.

Example:

```release-note:TYPE
Release note
```

If you do not require a release note to be included, please add the workflow/skip-changelog-entry label.

[jacobbednarz]

i think we can swap this for a d.GetOkExists and the value can use cloudflare.BoolPtr to take the boolean and return a *bool.

[broswen]

It looks like d.GetOkExists is deprecated and might be removed. Would just using d.GetOk be sufficient?

[jacobbednarz]

acceptance tests are passing locally

TF_ACC=1 go test $(go list ./...) -v -run "^TestAccCloudflareWaitingRoomEvent_Create" -count 1 -parallel 1 -timeout 120m -parallel 1
?   	github.com/cloudflare/terraform-provider-cloudflare	[no test files]
=== RUN   TestAccCloudflareWaitingRoomEvent_Create
=== PAUSE TestAccCloudflareWaitingRoomEvent_Create
=== CONT  TestAccCloudflareWaitingRoomEvent_Create
--- PASS: TestAccCloudflareWaitingRoomEvent_Create (12.98s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/cloudflare	13.359s
?   	github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/changelog-check	[no test files]
?   	github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/maintainer-only-file-check	[no test files]
?   	github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/tf-log-check	[no test files]
?   	github.com/cloudflare/terraform-provider-cloudflare/version	[no test files]

thank you for the solid effort here @broswen