pathlib.Path, PathLike and __fspath__ support (PEP 519)

It would be nice to support `pathlib.Path`-like objects, as per PEP 519: https://www.python.org/dev/peps/pep-0519/

Currently, the following fail with: TypeError: file_path_prefix must be a string or ValueProvider;got PosixPath('/tmp/pytest-of-forge-00/pytest-0/test_ptransform_beam0/output0.txt') instead

import pathlib

path = pathlib.Path('/path/to/home')
beam.io.WriteToText(path)

Raised by FileBasedSink:

class FileBasedSink(iobase.Sink):

  def __init__(self)
    if not isinstance(file_path_prefix, ((str, unicode), ValueProvider)):
      raise TypeError(...)

It could be implemented by calling os.fspath(file_path_prefix)