[Python][Dataset] Improve ergonomics of FileFragment

[asfimport]

FileFragment can be made more directly useful by adding convenience methods.

For example, a FileFragment could allow underlying file/buffer to be opened directly:

    def open(self):
        """
        Open a NativeFile of the buffer or file viewed by this fragment.
        """
        cdef:
            CFileSystem* c_filesystem
            shared_ptr[CRandomAccessFile] opened
            NativeFile out = NativeFile()

        buf = self.buffer
        if buf is not None:
            return pa.io.BufferReader(buf)

        with nogil:
            c_filesystem = self.file_fragment.source().filesystem()
            opened = GetResultValue(c_filesystem.OpenInputFile(
                self.file_fragment.source().path()))

        out.set_random_access_file(opened)
        out.is_readable = True
        return out

Additionally, a ParquetFileFragment's metadata could be introspectable:

    @property
    def metadata(self):
        from pyarrow._parquet import ParquetReader
        reader = ParquetReader()
        reader.open(self.open())
        return reader.metadata

Reporter: Ben Kietzman / @bkietz
Assignee: Miles Granger / @milesgranger

PRs and other links:

• GitHub Pull Request #14301

_{Note: This issue was originally created as ARROW-8201. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
@milesgranger  Perhaps you would be interested in finding out whether this issue still applies, and if so, to come up with a PR?

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
Issue resolved by pull request 14301
#14301