Provide `ignore_unknown_fields=True` option in `from_json` and `from_dict`

[tswast]

This blocking PR googleapis/python-bigquery#332 (and as a result the type to type_ change googleapis/python-bigquery#316 and googleapis/python-bigquery#328 with the new protobuf changes from the last year of updates from the BQML API -- though I might hack that one to ignore the type to type_ change)

I'd quite like to actually use proto-plus in google-cloud-bigquery ( googleapis/python-bigquery#332 ) rather than the current _pb workaround, but I can't without this change.

BigQuery very often launches new model types and model stats properties and updates the protos only after customers have used it in their projects. We need to be able to ignore unknown fields (and ideally unknown enums too, but that doesn't appear to be supported yet in the Python protobuf package protocolbuffers/protobuf#7392)

[software-dov]

N.B.: It turns out ignoring unknown enums is already possible iff the unknown enum variant is passed as an integer.

[tswast]

That's good, but unfortunately the API send enums as strings.

[tswast]

That's good, but unfortunately the API send enums as strings.

I wouldn't make that a blocker, though since it's an issue in the underlying protobuf library too