Bug 22144: Add method metadata() to Koha::Biblio

Bug 22144: Add method metadata() to Koha::Biblio

It would be very convenient to be able to retrieve a bib's attached metadata objects directly from the biblio object. This would be very useful for Template Toolkit syntax in slips and notices where a library would like to use arbitrary metadata fields that aren't stored in the database.

Test Plan:
1) Apply this patch
2) prove t/db_dependent/Koha/Biblio.t

Signed-off-by: Te Rahui Tunua <terahuitunua@catalyst.net.nz>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>