Bug 22700: Make biblio_metadata prefetchable from Koha::Biblio

Bug 22700: Make biblio_metadata prefetchable from Koha::Biblio

This patch makes $biblio->metadata be prefetchable as the Coding
guidelines require.

A new has_one relationship is added in the Biblio schema, named as the
accessor to also convey to proposed guidelines for readability.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Biblio.t
=> SUCCESS: No regressions!
- Sign off :-D

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>