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>

docs/data-sources/sqlserverflexalpha_user.md

@@ -0,0 +1,46 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackitprivatepreview_sqlserverflexalpha_user Data Source - stackitprivatepreview"
+subcategory: ""
+description: |-
+  SQLServer Flex user data source schema. Must have a region specified in the provider configuration.
+---
+
+# stackitprivatepreview_sqlserverflexalpha_user (Data Source)
+
+SQLServer Flex user data source schema. Must have a `region` specified in the provider configuration.
+
+## Example Usage
+
+```terraform
+# Copyright (c) STACKIT
+
+data "stackitprivatepreview_sqlserverflexalpha_user" "example" {
+  project_id  = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  instance_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  user_id     = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `instance_id` (String) ID of the SQLServer Flex instance.
+- `project_id` (String) STACKIT project ID to which the instance is associated.
+- `user_id` (Number) User ID.
+
+### Optional
+
+- `region` (String) The resource region. If not defined, the provider region is used.
+
+### Read-Only
+
+- `default_database` (String)
+- `host` (String)
+- `id` (String) Terraform's internal data source. ID. It is structured as "`project_id`,`region`,`instance_id`,`user_id`".
+- `port` (Number)
+- `roles` (Set of String) Database access levels for the user.
+- `status` (String)
+- `username` (String) Username of the SQLServer Flex instance.