BigQueryClient read_session: `row_restriction` is unused

[megaserg]

Looks like row_restriction parameter is saved in Python class but never propagated to actual BigQuery client. Is my understanding correct? @vlasenkoalexey

[vlasenkoalexey]

Indeed, looks like I missed to pipe it through to the C++ code. Will fix this.

[megaserg]

Thank you very much! Appreciate the convenient interface.