feat: Allow move state on new Observability resources

[joaopalet]

Draft PR to allow state move from old Argus to new Observability resources

• Follows instructions the official Terraform documentation but does not yet work
• Still gives error that provider "does not support moved operations across different resource types"

Tip

Workaround for users who want to migrate their Argus instance to Observability instance

1. Create the new stackit_observability_instance in the config (should have same fields and values as the old Argus one)
2. Import the existing instance to the new stackit_observability_instance resource
   • $ terraform import stackit_observability_instance.example <PROJECT_ID>,<INSTANCE_ID>
3. Delete the old Argus instance resource from the state
   • $ terraform state rm stackit_argus_instance.<INSTANCE_RESOURCE_NAME>

Related to #532

[marceljk]

Moving a argus resource to observability works. Descriptions of the argus resources extended where it's explained how to do it

[bahkauv70]

Looks good to me, test worked flawlessly. One minor thing only: Would it make sense to add examples of the move blocks from the terraform files to the documentation of the deprecated argus resources?

[marceljk]

Looks good to me, test worked flawlessly. One minor thing only: Would it make sense to add examples of the move blocks from the terraform files to the documentation of the deprecated argus resources?

@bahkauv70 Yes, I think it's a good idea. I'll add some examples to the documentation