Bug 7992: New plugin stocknumberAV

Bug 7992: New plugin stocknumberAV

Bugzilla description (from delaye):
1 / Create a list of authorised values(INVENTORY_NUMBER). In which the
code is the prefix and the description the last number assigned.

2/A space between the prefix and number.

3 / From a holding record, after indicate the prefix when you click on
the plugin -> koha search the last number from authorised values
assigned in the list and increment n +1 level meter

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Works as advertised (see provisos below). Tested the following:
1) INVENTORY category has only authorised value 'PR' => PR prefix filled
   in, everything else gives an error.
2) INVENTORY category has two authorised values, both work.
3) INVENTORY category has no authorised values, nothing is incremented
4) Barcode contains numbers, nothing is changed.
Two potential gotchas (neither of them significant enough to warrant
changing, in my opinion):
1) Prefixes must consist only of uppercase alphabetical characters.
2) Negative numbers are padded with zeros to the left of the negative
   sign.

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>