Implement PostgreSQL Flex database resource and data source (#453)

* Implement db resource

* Implement db data source

* Extend acc test

* Improve logs in other resources

* Add examples

* Generate docs

* Fix linter

docs/resources/postgresflex_database.md

@@ -0,0 +1,37 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_postgresflex_database Resource - stackit"
+subcategory: ""
+description: |-
+  Postgres Flex database resource schema. Must have a region specified in the provider configuration.
+---
+
+# stackit_postgresflex_database (Resource)
+
+Postgres Flex database resource schema. Must have a `region` specified in the provider configuration.
+
+## Example Usage
+
+```terraform
+resource "stackit_postgresflex_database" "example" {
+  project_id  = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  instance_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  name        = "mydb"
+  owner       = "myusername"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `instance_id` (String) ID of the Postgres Flex instance.
+- `name` (String) Database name.
+- `owner` (String) Username of the database owner.
+- `project_id` (String) STACKIT project ID to which the instance is associated.
+
+### Read-Only
+
+- `database_id` (String) Database ID.
+- `id` (String) Terraform's internal resource ID. It is structured as "`project_id`,`instance_id`,`database_id`".