Platform meshstack_integration.tf should include variables to override availability

[JohannesRudolph]

The STACKIT platform meshstack_integration.tf comes with this

resource "meshstack_platform" "stackit" {
  spec = {

   #...

    availability = {
      restriction              = "PRIVATE"
      publication_state        = "UNPUBLISHED"
      restricted_to_workspaces = [var.meshstack.owning_workspace_identifier]
    }

Of course platform engineers will want to start with the platform unpublished, but later request publishing it.
For building block definitions we have settled on the bbd_draft pattern, I think we need a similar facility for platform level integration modules as well.