bug_7458: A call number plugin

bug_7458: A call number plugin

Signed-off-by: Melia Meggs <melia@test.bywatersolutions.com>
A library using an inhouse callnumbering system wants to be able to generate the next available call number not already in use.  This custom call number plugin returns the next callnumber not already in use once a correct prefix has been entered.  The prefixes are four to six letters, space, then one or two numbers.
(cherry picked from commit ecc1e94f35491ba392aa3bfd17118c55fa14e55b)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>