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

stackit/internal/services/iaas/affinitygroup/const.go

@@ -0,0 +1,41 @@
+package affinitygroup
+
+const exampleUsageWithServer = `
+
+### Usage with server` + "\n" +
+	"```terraform" + `
+resource "stackit_affinity_group" "affinity-group" {
+  project_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  name       = "example-key-pair"
+  policy     = "soft-affinity"
+}
+
+resource "stackit_server" "example-server" {
+  project_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  name       = "example-server"
+  boot_volume = {
+    size        = 64
+    source_type = "image"
+    source_id   = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  }
+  affinity_group    = stackit_affinity_group.affinity-group.affinity_group_id
+  availability_zone = "eu01-1"
+  machine_type      = "g1.1"
+}
+` + "\n```"
+
+const policies = `
+
+### Policies
+
+* ` + "`hard-affinity`" + `- All servers launched in this group will be hosted on the same compute node.
+
+* ` + "`hard-anti-affinity`" + `- All servers launched in this group will be
+    hosted on different compute nodes.
+
+* ` + "`soft-affinity`" + `- All servers launched in this group will be hosted
+    on the same compute node if possible, but if not possible they still will be scheduled instead of failure.
+
+* ` + "`soft-anti-affinity`" + `- All servers launched in this group will be hosted on different compute nodes if possible,
+	but if not possible they still will be scheduled instead of failure.
+`