Bug 31791: Add Koha::Biblio::Metadata->record_source

Bug 31791: Add Koha::Biblio::Metadata->record_source

This patch adds a method for retrieving the related *Koha::RecordSource* from a *Koha::Biblio::Metadata* object.

The method is covered by tests:

1. Apply this patch
2. Run:
   $ ktd --shell
  k$ prove t/db_dependent/Koha/Biblio/Metadata.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Sponsored-by: ByWater Solutions
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>