Bug 20624: Net::OAuth2::AuthorizationServer is not a hard dependency

Bug 20624: Net::OAuth2::AuthorizationServer is not a hard dependency

While we get packaging sorted, Net::OAuth2::AuthorizationServer is not a
hard dependency for Koha and the feature requiring it is disabled by
default.

This patch:
- Makes the dependency optional
- Makes the unit tests for the OAuth2 client credentials flow skip if
  the dependency is not met.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>