[IMP] tools: simplify and modernize file_open()

- Remove legacy arguments (`subdir`, `pathinfo`) of `file_open()`,
  they were not used anymore and made the code more complicated

- Drop the long-deprecated support for files inside zip archives,
  considering that zipped modules were discontinued a long time ago:
  278ed718

- Remove the redundant `_file_open()` method, that should never have been
  called directly anyway

- Add the possibility to filter allowed file extensions, in order to
  avoid opening up access to arbitrary addons files, such as .py files,
  depending on the context of use

- Split up the verification of the file path, to make it accessible
  without actually opening the file, as a separate `file_path()` function.

closes odoo/odoo#68043

Signed-off-by: Olivier Dony (odo) <odo@openerp.com>