docs: adds snippet for creating table with external data config

[aribray]

Adds snippet and test for create_table_external_data_configuration

• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #773 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• docs/usage/tables.rst:67, tag bigquery_create_table_external_data_configuration
• samples/create_table_external_data_configuration.py:25, tag bigquery_create_table_external_data_configuration
• samples/create_table_external_data_configuration.py:26, tag bigquery_create_external_table_definition

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