Add new resource and datasource for Load Balancer credentials (#255)

* Implement and test credential resource and data source

* Fix descriptions in load balancer instance schema

* Extend acceptance tests

* Add acceptance test requirements to README

* Generate updated docs

* Fix linter issues

* Add examples and update docs

* Fix examples

* Improvements from review

* Remove credential data source

docs/data-sources/loadbalancer.md

@@ -3,12 +3,12 @@
 page_title: "stackit_loadbalancer Data Source - stackit"
 subcategory: ""
 description: |-
-  Load Balancer resource schema.
+  Load Balancer data source schema. Must have a region specified in the provider configuration.
 ---
 
 # stackit_loadbalancer (Data Source)
 
-Load Balancer resource schema.
+Load Balancer data source schema. Must have a `region` specified in the provider configuration.
 
 ## Example Usage
 

docs/resources/loadbalancer_credential.md

@@ -0,0 +1,37 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_loadbalancer_credential Resource - stackit"
+subcategory: ""
+description: |-
+  Load balancer credential resource schema. Must have a region specified in the provider configuration.
+---
+
+# stackit_loadbalancer_credential (Resource)
+
+Load balancer credential resource schema. Must have a `region` specified in the provider configuration.
+
+## Example Usage
+
+```terraform
+resource "stackit_loadbalancer_credential" "example" {
+  project_id   = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  display_name = "example-credentials"
+  username     = "example-user"
+  password     = "example-password"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `display_name` (String) Credential name.
+- `password` (String) The password used for the ARGUS instance.
+- `project_id` (String) STACKIT project ID to which the load balancer credential is associated.
+- `username` (String) The username used for the ARGUS instance.
+
+### Read-Only
+
+- `credentials_ref` (String) The credentials reference can be used for observability of the Load Balancer.
+- `id` (String) Terraform's internal resource ID. It is structured as "`project_id`","`credentials_ref`".