Bug 31503: Make opac-patron-consent more generic

Bug 31503: Make opac-patron-consent more generic

Adds $patron->consent and $consents->available_types.
Incorporates them into script/template.
Provides two unit tests.

Note: A follow-up patch helps you test this with an
example plugin.

Test plan:
Run t/db_dependent/Koha/Patron.t
Run t/db_dependent/Koha/Patron/Consents.t
Toggle the value of pref PrivacyPolicyConsent and look at
OPAC account, tab Consents.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>