[Fix] Returns an Error if the Grafana-Terraform-Provider Header is missing for Requests to the SLO API

[elainevuong]

To prevent regression due to missing headers from the Terraform Provider, we add a check directly in the TF Provider that ensures that resourceSloRead calls always have a readOnly.Provenance field of "terraform".

resourceSloRead is called after all CREATE/UPDATEs made by the Terraform Provider, so it is reasonable to expect that the modified Slo would have a readOnly.Provenance field of terraform.

If this is missing, it's likely because the header was removed from within the Terraform Provider.