feat: Add ExternalRuntimeOptions to BigQuery routine

This change introduces the `ExternalRuntimeOptions` class to the
`google.cloud.bigquery.routine` module, allowing users to configure
runtime options for external routines.

Key changes:
- Created the `ExternalRuntimeOptions` class with setters and getters for
  `container_memory`, `container_cpu`, `runtime_connection`,
  `max_batching_rows`, and `runtime_version`.
- Updated the `Routine` class to include an `external_runtime_options`
  property that accepts an `ExternalRuntimeOptions` object.
- Added comprehensive unit tests for the new class and its integration
  with the `Routine` class, including tests for both valid and invalid
  input values.

Author: google-labs-jules[bot]