Bug 29869: Add Koha::Hold->fill

Bug 29869: Add Koha::Hold->fill

This patch introduces a new method for marking a hold as filled. The
code is the result of tracking the following methods in C4::Reserves:
- ModReserveFill
- GetReserveFee
- ChargeReserveFee

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Hold.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>