Ability to charge connected ILS account for printing funds

[sekjal]

Is your feature request related to a problem? Please describe.
Right now, the only supported ways to put add printing funds to a Libki account are either use a Jamex coin-op machine with the Print Station component, or to pay a staff member who then manually adds the funds. Problems with this are:

• Not every library has access to Jamex hardware
• Not every library handles cash at all times
• Not every patron carries sufficient cash with them
• Not every patron wants to interact with human to complete their print job

Describe the solution you'd like
For users with an ILS-backed account (i.e. created by SIP2), add the ability charge printing funds to their library account, with the following limitations:

• only works for SIP accounts, as there is not guaranteed shared key for local or LDAP accounts to use with the ILS (if there is an ILS at all!)
• the library can configure set amounts to choose from
• a maximum outstanding balance limit shall be configurable within Libki; if a user's ILS balance is greater than this amount, they will not be able to add additional funds through this method
• each ILS will need a separate driver, as there is not standard implementation for charging financial transactions. Koha will be used as the default at first, with Evergreen and FOLIO to follow. Proprietary ILS connections can be added as user need reflects

This will allow patrons to pay later for printing, and using the mechanisms the library already has in place to handle overdues, lost item fees, etc., leveraging the existing trusted relationship they already have with the library.

Describe alternatives you've considered
SIP2 can submit fine payments, but not issue new charges. Negative amounts are not accepted.

While the NCIP protocol does have fiscal transactions are part of the spec, it is outside of the 'core' set of functions, and does not seem to have been implemented in any ILS.

Additional context
A second-wave refinement could be to set different max outstanding balances for different patron categories. Use cases:

• children have a max outstanding limit of $0.00, meaning they cannot charge printing funds this way
• graduate students or faculty have a higher limit than undergraduate students

This could be implemented as part of the existing Advanced Settings model.