Bug 35073: Rename additional_fields to available_additional_fields

Bug 35073: Rename additional_fields to available_additional_fields

'additional_fields' is ambiguous and in other places we name this available_additional_fields instead. Do the same here.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 05d131febf2d98d1c0ec18da845589dcd80a5e8d)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit a5fa92d2c0cd30a982660e427285a22b8f1a8977)
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>