Allow ParquetIO.read/parse without explicit schema

Parquet files store their schema, so it could make sense for users to want to read a PCollection<GenericRecord> or PCollection<Row> without needing to pass explicitly the Schema. e.g. ParquetIO.read().from(...).

Request from the user mailing list:
https://lists.apache.org/thread.html/r6fd6c583383c950766a74100d8f923aa9dab39b2c47524a6bbd08508%40%3Cuser.beam.apache.org%3E