arrow

GH-38857: [Python] Add append mode for pyarrow.OsFile (#38820)

### Rationale for this change

Seems reasonable. :) 

### What changes are included in this PR?

Exposes the C++ append parameter from FileOutputStream  to PyArrow's OSFile.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Can add `a` or `ab` to `mode` parameter in `pyarrow.OsFile`

* Closes: #38857

Authored-by: Miles Granger <miles59923@gmail.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>