Onboard SQLServer Flex user resource (#403)

* Onboard SQLServer Flex user resource

* change roles

* fix unit tests

* make database field optional // adapt test and documentation

* add sleep time to instance creation

* fix service name in logs and descriptions

* extend username plan modifiers

* update docs

* remove database field

* remove database // make roles optional

* update docs

docs/data-sources/sqlserverflex_user.md

@@ -0,0 +1,38 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_sqlserverflex_user Data Source - stackit"
+subcategory: ""
+description: |-
+  SQLServer Flex user data source schema. Must have a region specified in the provider configuration.
+---
+
+# stackit_sqlserverflex_user (Data Source)
+
+SQLServer Flex user data source schema. Must have a `region` specified in the provider configuration.
+
+## Example Usage
+
+```terraform
+data "stackit_sqlserverflex_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` (String) User ID.
+
+### Read-Only
+
+- `host` (String)
+- `id` (String) Terraform's internal data source. ID. It is structured as "`project_id`,`instance_id`,`user_id`".
+- `port` (Number)
+- `roles` (Set of String)
+- `username` (String)