Bug 27855: Implement Koha::Patron::Attribute->to_api_mapping

Bug 27855: Implement Koha::Patron::Attribute->to_api_mapping

This patch introduces a mapping to render extended attributes on the
API. As they have an ID, and they will generally be used on the
/patrons umbrella, I removed the borrowernumber on the mapping.

Another option would be to add the field, but make it optional. It
really feels redundant, so I take it out for now.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>