Bug 19173: Make OPAC online payments pluggable

Bug 19173: Make OPAC online payments pluggable

While PayPal is fairly universal, there is a plethora of online
payment system that are far more localized, servicing a single
country ( e.g. Bug 18968 ) or even a single  city! Instead of
adding support for each and every one of these payment options
directly into Koha, it makes more sense to add the ability to
create online payment plugins.

Test Plan:
1) Apply this patch
2) Download and install the Kitchen Sink plugin version 2.1.1 or later
   https://github.com/bywatersolutions/koha-plugin-kitchen-sink/releases
3) In the plugin options, enable the opac payments option
4) Create a patron with one or more fines
5) Log into the opac as that patron, note you now have the option
   to pay online via KitchenSink ImaginaryPay
6) Make an online payment
7) Note the payment was processed correctly

Sponsored-by: Washoe County Library System
Signed-off-by: Kyle M Hall <kyle@gmail.com>
Signed-off-by: Magnus Enger <magnus@libriotech.no>
Awesome enhancement! I know we want to add at least one Norwegian
payment service at some point.
I followed the test plan and everything works as advertised. Turning
off the "opac payments option" makes the option dissappear cleanly
from the OPAC. I have *not* looked at the code or done any
considerations about security.
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>