Add a "default labels" setting to the Terraform povider

Similar to the AWS provider's default_tags provider config, the Spacelift provider should have a default_labels setting that would automatically attach a set of labels to all resources created by it (those that can be labelled, at least).

Also submitted as an issue to the provider repo:

https://github.com/spacelift-io/terraform-provider-spacelift/issues/606