[Fix] Adds check to ensure that the Grafana-Terraform-Provider header is set

[elainevuong]

To prevent regression due to missing request headers from the Terraform Provider, we add a check in the TF Provider Tests that ensures that following a successful CREATE or UPDATE, that a SLO returned from the READ call to the SLO API has a provenance field of terraform.

This is done directly in the TF Provider Tests resource_slo_test and does NOT affect the actual function of the Terraform Provider, which means it does not interfere with IMPORT functionality.

Fixes #1630

[github-actions]

In order to lower resource usage and have a faster runtime, PRs will not run Cloud tests automatically.
To do so, a Grafana Labs employee must trigger the cloud acceptance tests workflow manually.

[julienduchesne]

This prevents someone from importing a SLO and taking it over with Terraform

[ellisda]

This prevents someone from importing a SLO and taking it over with Terraform

Is that because the resourceSloRead function is called during import as well? Is there a unit or integration test that we can add such a check to, outside of the production code path?

[julienduchesne]

Yep, read is called on imports. The only way to test this would be to have a new acceptance test that creates a SLO without the header and then imports it (plenty of import examples in other TF tests)

[elainevuong]

I'll update this - and add the test @julienduchesne suggested - a new acceptance test that creates a SLO without the header and then imports it. thanks for the comments!

ended up doing this check directly in the TF Provider Tests, and not in the Provider itself so it shouldn't affect the Import functionality. doing the import tests took a bit more time to think through - I think it would require changes to the TF SLO Schema, since we don't currently have the readOnly field included in our SLO Schema anywhere. decided to just test for the header directly in our TF Provider Tests instead