Feature: CDN custom domain resource and data source (#801)

* Feature: CDN custom domain resource and data source

* stabilize acceptance tests

* add guide

* review changes

---------

Co-authored-by: Malte Ehrlen <malte.ehrlen@freiheit.com>

docs/data-sources/cdn_custom_domain.md

@@ -0,0 +1,39 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_cdn_custom_domain Data Source - stackit"
+subcategory: ""
+description: |-
+  CDN distribution data source schema.
+  ~> This resource is in beta and may be subject to breaking changes in the future. Use with caution. See our guide https://registry.terraform.io/providers/stackitcloud/stackit/latest/docs/guides/opting_into_beta_resources for how to opt-in to use beta resources.
+---
+
+# stackit_cdn_custom_domain (Data Source)
+
+CDN distribution data source schema.
+
+~> This resource is in beta and may be subject to breaking changes in the future. Use with caution. See our [guide](https://registry.terraform.io/providers/stackitcloud/stackit/latest/docs/guides/opting_into_beta_resources) for how to opt-in to use beta resources.
+
+## Example Usage
+
+```terraform
+data "stackit_cdn_custom_domain" "example" {
+  project_id      = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  distribution_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  name            = "https://xxx.xxx"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `distribution_id` (String) CDN distribution ID
+- `name` (String)
+- `project_id` (String) STACKIT project ID associated with the distribution
+
+### Read-Only
+
+- `errors` (List of String) List of distribution errors
+- `id` (String) Terraform's internal resource identifier. It is structured as "`project_id`,`distribution_id`".
+- `status` (String) Status of the distribution