[PROPOSAL] ParquetIO support for Python SDK

[Heejong Lee]

Hi,

I'm working on BEAM-4444: Parquet IO for Python SDK.

Issue: https://issues.apache.org/jira/browse/BEAM-4444
Design doc:
https://docs.google.com/document/d/1-FT6zmjYhYFWXL8aDM5mNeiUnZdKnnB021zTo4S-0Wg
WIP PR: https://github.com/apache/beam/pull/6763

Any feedback is appreciated. Thanks!