Change client_options type for PredictionServiceAsyncClient

[igor-drozdov]

• Make it consistent with the doc string
• Make it consistent with PredictionServiceClient:

  python-aiplatform/google/cloud/aiplatform_v1/services/prediction_service/client.py

  Line 636 in 547bf01

  client_options: Optional[Union[client_options_lib.ClientOptions, dict]] = None,

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Fixes #<issue_number_goes_here> 🦕