External diff for `pipeline_config` not detected

[tgoodsell-tempus]

Since pipeline_config is not read from the API during the Read operation, a diff for it is not picked up when manually changing the pipeline configuration using fly.

Reproducing:

1. Create pipeline with Terraform, in this example named my-test-pipeline.
2. fly get-pipeline -p my-test-pipeline > ~/testing.yaml
3. Edit testing.yaml with your changes
4. fly set-pipeline -p my-test-pipeline -c ~/test.yaml
5. Run terraform plan or terraform refresh and see no differences detected