feat: Onboard affinity groups resource and data source (#652)

* onboard affinity_groups resource and data source
- add tests and descriptions
- fix: server doesn't use affinity_group value for payload

* Update descriptions

docs/data-sources/affinity_group.md

@@ -0,0 +1,35 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_affinity_group Data Source - stackit"
+subcategory: ""
+description: |-
+  Affinity Group schema. Must have a region specified in the provider configuration.
+---
+
+# stackit_affinity_group (Data Source)
+
+Affinity Group schema. Must have a `region` specified in the provider configuration.
+
+## Example Usage
+
+```terraform
+data "stackit_affinity_group" "example" {
+  project_id        = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  affinity_group_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `affinity_group_id` (String) The affinity group ID.
+- `project_id` (String) STACKIT Project ID to which the affinity group is associated.
+
+### Read-Only
+
+- `id` (String) Terraform's internal resource identifier. It is structured as "`project_id`,`affinity_group_id`".
+- `members` (List of String) Affinity Group schema. Must have a `region` specified in the provider configuration.
+- `name` (String) The name of the affinity group.
+- `policy` (String) The policy of the affinity group.