fdo#77051: Preservation of Index field flag '\f'.

Problem Description:
- In LibreOffice, the index field flag '\f' was not
  getting preserved after roundtrip as there was no
  support for it.
- '\f' field flag is used for Specific Entry Type.
  ex. In our case it is "Syn"

Implementation:
- Provided import & export support for Index field flag '\f'
  and added UT for the same.

Change-Id: I97c2456dd73c8bdf89ab105f8cac71bf7e2ad164
Reviewed-on: https://gerrit.libreoffice.org/8839
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>