Bug 17813: Add a primary key to borrower_attributes

Bug 17813: Add a primary key to borrower_attributes

This patch adds 'borrower_attributes' a field (if) which
will act as a primary key.

This is needed for DBIC to be used to handle rows, and also will help
when faced with the implementation of the REST api for this resource.

To test:
- Run all patron modification / attributes and verify nothing breaks
- Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>