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

examples/resources/stackit_loadbalancer_credential/resource.tf

@@ -0,0 +1,6 @@
+resource "stackit_loadbalancer_credential" "example" {
+  project_id   = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  display_name = "example-credentials"
+  username     = "example-user"
+  password     = "example-password"
+}