Add a cloudflare_zone_cache_variants resource

[maleblond]

We'd like to be able to manage cache variants through terraform so I added a.cloudflare_zone_cache_variants resource.

I added some unit tests + I did some manual testing. A cloudflare_zone_cache_variants resource would look like this:

resource "cloudflare_zone_cache_variants" "variants" {
  zone_id = <zone_id>
  gif     = ["image/avif", "image/webp", "image/gif"]
  webp    = ["image/whatever"]
  ...
}

It's my first time updating a terraform provider, so there may be some very ugly things in there not following the best practices 😅

[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]

not really sure how i feel about the dynamic nature of creating schema items due to making it difficult for discovery. i'll have a think about this.

[maleblond]

👍 My main goal was not to repeat the whole schema over and over again. An alternative would be to do something like:

"avif": extensionSchema(),
"gif": extensionSchema(),

I think an upside with this alternative is that the schema is a little bit easier to read. I'll do that, LMK what you think

[jacobbednarz]

given create and update are essentially the same funcitonality, you could point Create and Update here to the same method.

[maleblond]

👍 I merged both function

[jacobbednarz]

acceptance tests are green

TF_ACC=1 go test $(go list ./...) -v -run "^TestAccCloudflareZoneCacheVariants_" -count 1 -parallel 1 -timeout 120m -parallel 1
?   	github.com/cloudflare/terraform-provider-cloudflare	[no test files]
=== RUN   TestAccCloudflareZoneCacheVariants_OneExt
--- PASS: TestAccCloudflareZoneCacheVariants_OneExt (9.69s)
=== RUN   TestAccCloudflareZoneCacheVariants_AllExt
--- PASS: TestAccCloudflareZoneCacheVariants_AllExt (9.12s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/cloudflare	19.301s
?   	github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/changelog-check	[no test files]
?   	github.com/cloudflare/terraform-provider-cloudflare/version	[no test files]

really solid effort here @maleblond, your first contribution no less! thanks for such a solid PR.