feat: mssql alpha instance (#2)

* fix: remove unused attribute types and functions from backup models

* fix: update API client references to use sqlserverflexalpha package

* fix: update package references to use sqlserverflexalpha and modify user data source model

* fix: add sqlserverflexalpha user data source to provider

* fix: add sqlserverflexalpha user resource and update related functionality

* chore: add stackit_sqlserverflexalpha_user resource and instance_id variable

* fix: refactor sqlserverflexalpha user resource and enhance schema with status and default_database

---------

Co-authored-by: Andre Harms <andre.harms@stackit.cloud>
Co-authored-by: Marcel S. Henselin <marcel.henselin@stackit.cloud>

examples/resources/stackitprivatepreview_postgresflexalpha_instance/resource.tf

@@ -0,0 +1,24 @@
+# Copyright (c) STACKIT
+
+resource "stackitprivatepreview_postgresflexalpha_instance" "example" {
+  project_id      = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  name            = "example-instance"
+  acl             = ["XXX.XXX.XXX.X/XX", "XX.XXX.XX.X/XX"]
+  backup_schedule = "00 00 * * *"
+  flavor = {
+    cpu = 2
+    ram = 4
+  }
+  replicas = 3
+  storage = {
+    class = "class"
+    size  = 5
+  }
+  version = 14
+}
+
+# Only use the import statement, if you want to import an existing postgresflex instance
+import {
+  to = stackitprivatepreview_postgresflexalpha_instance.import-example
+  id = "${var.project_id},${var.region},${var.postgres_instance_id}"
+}