Importing a pipeline with `provider_settings` produces spurious diffs

[ellsclytn]

Description

When importing an existing buildkite_pipeline that has a provider_settings block in its Terraform configuration, terraform plan shows the full provider_settings object as needing to be created, even when the config matches the actual pipeline. Additionally, if even a single provider_settings attribute differs, all unspecified string attributes show spurious diffs like null -> (known after apply).

To Reproduce

1. Create a pipeline in Buildkite (or use an existing one)
2. Write a Terraform config for it with a provider_settings block:

   resource "buildkite_pipeline" "example" {
     name       = "my-pipeline"
     repository = "https://github.com/org/repo.git"
   
     provider_settings {
       build_branches      = true
       build_pull_requests = false
     }
   }

3. Run terraform import buildkite_pipeline.example <pipeline-id>
4. Run terraform plan
5. The plan shows the entire provider_settings block as a new addition, plus fields like:

   ~ filter_condition                         = null -> (known after apply)
   ~ pull_request_branch_filter_configuration = null -> (known after apply)
   ~ trigger_mode                             = null -> (known after apply)
   

Expected behavior

After importing, terraform plan should show no changes if the config matches the pipeline, and only the actually-differing attributes if the config differs.