Fix error details type on google_database_migration_service_migration_job

[modular-magician]

In the event that there is an error with a MigrationJob in GCP, the Terraform google provider can not properly create a plan because the schema for google_database_migration_service_migration_job is incorrect.

... expected type 'string', got unconvertible type 'map[string]interface {}' ...

I am not sure if this is the proper way to fix the issue, but based on the details of the error response from Terraform and comparing magic-modules source code with gcloud output, it seems like this code change will fix the issue. magic-modules has a MigrationJob error configured as a KeyValuePairs when it is in fact an Object with arbitrary properties.

Terraform execution error:

Error: Error reading MigrationJob: error.0.details.1.metadata: '' expected type 'string', got unconvertible type 'map[string]interface {}', value: 'map[errorMessage:The migration job is currently running. Restart is not available on a running Migration job. nativeErrorMessage:Cloud SQL replica is already setup and it's replicating from the source database server. severity:ERROR]'
  with google_database_migration_service_migration_job.dms_task,
  on main.tf line 167, in resource "google_database_migration_service_migration_job" "dms_task":
 167: resource "google_database_migration_service_migration_job" "dms_task" {

Data returned from gcloud database-migration migration-jobs describe dms-task:

...
error:
  code: 9
  details:
  - '@type': type.googleapis.com/google.cloud.clouddms.v1.MigrationJobVerificationError
    errorCode: CANT_RESTART_RUNNING_MIGRATION
    errorDetailMessage: Cloud SQL replica is already setup and it's replicating from
      the source database server.
    errorMessage: "The migration job is currently running. Restart is not available\
      \ on a running Migration job. Details: Cloud SQL replica is already setup and\
      \ it's replicating from the source database server."
  - '@type': type.googleapis.com/google.rpc.ErrorInfo
    domain: datamigration.googleapis.com
    metadata:
      errorMessage: The migration job is currently running. Restart is not available
        on a running Migration job.
      nativeErrorMessage: Cloud SQL replica is already setup and it's replicating
        from the source database server.
      severity: ERROR
    reason: CANT_RESTART_RUNNING_MIGRATION
  - '@type': type.googleapis.com/google.rpc.Help
    links:
    - description: 'Learn more:'
      url: https://cloud.google.com/database-migration/docs/diagnose-issues
  message: The migration job is currently running. Restart is not available on a running
    Migration job.
labels:
  goog-terraform-provisioned: 'true'
...

fix: error details type on google_database_migration_service_migration_job

Derived from GoogleCloudPlatform/magic-modules#12858