Implement key pair resource (#588)

* Revert "Revert "Implement key pair resource (#578)" (#581)"

This reverts commit 600847a2ea.

* feat: Update iaas SDK module version; Use beta API in key pair resource

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

@@ -0,0 +1,25 @@
+package keypair
+
+const exampleUsageWithServer = `
+
+### Usage with server` + "\n" +
+
+	"```terraform" + `
+resource "stackit_key_pair" "keypair" {
+  name       = "example-key-pair"
+  public_key = chomp(file("path/to/id_rsa.pub"))
+}
+
+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"
+  }
+  availability_zone = "eu01-1"
+  machine_type      = "g1.1"
+  keypair_name      = "example-key-pair"
+}
+`