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ARROW-5258: [C++/Python] Collect file metadata of dataset pieces

This PR supersedes PR #4166 and provides
- [x] file metadata API (C++)
- [x] ParquetWriter `metadata` property, only available after calling `close` method (Python)
- [x] FileMetaData `to_dict` method for collecting file metadata information to a dictionary (Python)
- [x] support for `metadata_collector` kw argument to `ParquetWriter` and `write_to_dataset` for collecting the file metadata instances of dataset pieces.
- [x] unit-test

Author: Pearu Peterson <pearu.peterson@gmail.com>

Closes #4236 from pearu/pearu/arrow-1983-2 and squashes the following commits:

11419a8de <Pearu Peterson> Remove unnecessary file_metadata_.reset calls.
d6c9f600b <Pearu Peterson> Fix lint error
6375d9cbe <Pearu Peterson> Expose file metadata to Python. Introduce metadata_collector kw argument to ParquetWriter, used to collect metadata instances of dataset pieces.
940b93505 <Pearu Peterson> Introduce ParquetFileWriter::metadata.