docs: add code sample for using BigQueryWriteClient with a compiled proto2 module

[tswast]

Follow-up to #269

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/append_rows_proto2.py:15, tag bigquerystorage_append_rows_raw_proto2
• samples/snippets/sample_data.proto:15, tag bigquerystorage_append_rows_raw_proto2_definition

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