[9.20] new: test: pytest helpers for dnssec and zone setup

- Create `isctest.zone.Zone` helper for zone setup (including signing).
- Add `ZoneKey` helpers for both dnssec-keygen managed keys and python-based keys.
- Add `dnssec_py` shared test setup for DNSSEC tests.
- Add the first example - refactor `nsec3_delegations` into a `dnssec_py` test module.

Backport of MR !11807

Merge branch 'backport-nicki/pytest-dnssec-py-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12191