Extend resource stackit_project to support labels

[roberth1988]

Since stackit supports setting labels and its internally used within the terraform provider, we should extend the resource to have a field labels with map(string) which enables us to provide custom label values to a project.

Expected format:

resource "stackit_project" "example" {
  name                          = "example"
  parent_container_id = "parent-contaier-id"
  billing_ref                  = var.project_billing_ref
  owner_email              = var.project_owner_email
  labels = {
    mylabelx = "my-value"
    mylabelz = "mynicevalue"
  }
}

There are some reserved labels:

• billingReference
• scope

which should not be set with the labels attribute.