cloudflare_fallback_domain support for default entries

[tjstansell]

This should close #1385 and adds two new options (enabled by default) to support the default entries provided by teams initially and to support restoring those entries on resource deletion. This updates the behavior to align with the docs and the intended behavior most folks expect, while providing control to customize the behavior.

[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]

thanks for the PR! i've gone through and added comments inline but this PR does raise some provider design questions which i don't think we've quite ironed out in the current state.

the two big things:

• restore_default_domains_on_delete as an attribute doesn't really make a whole heap of sense if it is the default state, and we return to that state upon no longer managing the resource. like zone settings, we can just set those values in the Delete operation back to the original state and leave it at that.
• the CustomizeDiff behaviour seems to be trying to do too much implicitly for the end user at the risk of introducing bugs through the types weirdness and incompatibilities. i would recommend a documentation update over this approach mentioning the values need to be explicitly provided should end users wish to include any of the default domains (favouring explicitness in managing the resource).

[tjstansell]

OK. I've updated this PR to simplify everything. It now will reset the default entries on delete, always. It also updates the documentation to explicitly show how to add entries to the default list. If you hear from the service teams on being able to support an API endpoint to reset defaults, we can change to that. I guess it'd be up to you on if you'd rather wait for that or do this now and then update once it's available.

[jacobbednarz]

pending cloudflare/cloudflare-go#784