Add sqlserverflex instance (#381)

* Draft implementation sqlserverflex instance

* Finish implementation

* Fix acc test

* Changes after review

docs/data-sources/sqlserverflex_instance.md

@@ -0,0 +1,61 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_sqlserverflex_instance Data Source - stackit"
+subcategory: ""
+description: |-
+  MongoDB Flex instance data source schema. Must have a region specified in the provider configuration.
+---
+
+# stackit_sqlserverflex_instance (Data Source)
+
+MongoDB Flex instance data source schema. Must have a `region` specified in the provider configuration.
+
+
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `instance_id` (String) ID of the MongoDB Flex instance.
+- `project_id` (String) STACKIT project ID to which the instance is associated.
+
+### Read-Only
+
+- `acl` (List of String) The Access Control List (ACL) for the MongoDB Flex instance.
+- `backup_schedule` (String) The backup schedule. Should follow the cron scheduling system format (e.g. "0 0 * * *").
+- `flavor` (Attributes) (see [below for nested schema](#nestedatt--flavor))
+- `id` (String) Terraform's internal data source. ID. It is structured as "`project_id`,`instance_id`".
+- `name` (String) Instance name.
+- `options` (Attributes) Custom parameters for the MongoDB Flex instance. (see [below for nested schema](#nestedatt--options))
+- `replicas` (Number)
+- `storage` (Attributes) (see [below for nested schema](#nestedatt--storage))
+- `version` (String)
+
+<a id="nestedatt--flavor"></a>
+### Nested Schema for `flavor`
+
+Read-Only:
+
+- `cpu` (Number)
+- `description` (String)
+- `id` (String)
+- `ram` (Number)
+
+
+<a id="nestedatt--options"></a>
+### Nested Schema for `options`
+
+Read-Only:
+
+- `edition` (String)
+- `retention_days` (Number)
+
+
+<a id="nestedatt--storage"></a>
+### Nested Schema for `storage`
+
+Read-Only:
+
+- `class` (String)
+- `size` (Number)