fix: raise error if inserting rows with unknown fields

[plamut]

Fixes #151.

This PR makes sure that unknown fields are not silently ignored when inserting rows, but instead such errors are detected and raised loudly.

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[shollyman]

I think I'm missing something here. The user controls whether they want this behavior in tabledata.insertall via ignoreUnknownValues, which is present in the insert_rows_json() method signature, but I don't see how that's plumbed to this helper.

[plamut]

@shollyman Hmm, good point. I'll check how the helper can support the ignore_unknown_values parameter.

[plamut]

Changed the helper to include missing fields and let the backend handle that.

The ignore_unknown_values argument to user-facing methods in the client is orthogonal to that, thus the helper signature didn't need a change (FWIW, the backend honors the argument, checked it).

[plamut]

Just the coverage failure it seems, will cover the missed code path.

[tseaver]

Assuming that the Python3 test failures in test_download_arrow_tabledata_list_unknown_field_type due to a pyarrow release, I've kicked off a merge.

[plamut]

Indeed, and those tests were updated in https://github.com/googleapis/python-bigquery/pull/192/files