Add a Hadoop FileSystem implementation of Beam's FileSystem

Beam's FileSystem creates an abstraction for reading from files in many different places.

We should add a Hadoop FileSystem implementation (https://hadoop.apache.org/docs/r2.8.0/api/org/apache/hadoop/fs/FileSystem.html) - that would enable us to read from any file system that implements FileSystem (including HDFS, azure, s3, etc..)

I'm investigating this now.