Bug 37513: Add Koha::RecordSource->usage_count

Bug 37513: Add Koha::RecordSource->usage_count

This patch adds the `usage_count` method to be used for embedding from
the API. Tests cover all use cases.

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

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>