Add WIF authentication fields to dbtcloud_bigquery_credential

[mariusostby]

Is your feature request related to a problem? Please describe.

When managing BigQuery connections with Workload Identity Federation (WIF), the dbtcloud_global_connection resource supports WIF configuration, but dbtcloud_bigquery_credential does not expose the credential-level fields required to complete WIF setup.
This means credentials created via Terraform default to service account JSON auth, even when the connection is configured for WIF. The credential must be manually patched via direct API calls to enable WIF authentication.

Describe the solution you'd like

Add WIF-related fields to dbtcloud_bigquery_credential:

• auth_type — authentication method, one of service-account-json, oauth-secrets, or external-oauth-wif
• workload_pool_provider_path — the fully qualified WIF provider resource name (e.g., //iam.googleapis.com/projects/123/locations/global/workloadIdentityPools/my-pool/providers/my-provider)
  These map to the credential_details.fields structure in the v3 API:

{
  "credential_details": {
    "fields": {
      "auth_type": { "value": "external-oauth-wif" },
      "workload_pool_provider_path": { "value": "//iam.googleapis.com/..." }
    }
  }
}

Describe alternatives you've considered

Direct API calls via shell script — We currently run an idempotent shell script after terraform apply that POSTs to /api/v3/accounts/{account_id}/projects/{project_id}/credentials/{credential_id}/ with the full credential_details.fields payload. This works but breaks the Terraform-as-source-of-truth model and adds operational complexity.
dbtcloud_extended_attributes — We tried attaching auth_type and workload_pool_provider_path via extended attributes on the profile, but these map to profiles.yml runtime keys and do not set the underlying credential's auth_type in the API.

Additional context

Related: #584 added WIF support to dbtcloud_global_connection. This request completes WIF support on the credential side.