feat: Allow managing server state in stackit_server resource (#623)

* feat: implement state switching in resource

* chore: fix linter issues

* feat: fix testcases

* chore: update documentation

* feat: replace backoff implementation with canonical wait functionality

* feat: refactor update method to correctly handle state changes of shelved servers

* chore: reverted documentation changes

* feat: updated server documentation

* feat: configured desired_state as "write-only" attribute

* feat: update to command help

docs/resources/server.md

@@ -384,6 +384,7 @@ resource "stackit_server" "user-data-from-file" {
 - `affinity_group` (String) The affinity group the server is assigned to.
 - `availability_zone` (String) The availability zone of the server.
 - `boot_volume` (Attributes) The boot volume for the server (see [below for nested schema](#nestedatt--boot_volume))
+- `desired_status` (String) The desired status of the server resource. Defaults to 'active' Supported values are: `active`, `inactive`, `deallocated`.
 - `image_id` (String) The image ID to be used for an ephemeral disk on the server.
 - `keypair_name` (String) The name of the keypair used during server creation.
 - `labels` (Map of String) Labels are key-value string pairs which can be attached to a resource container