[9.20] new: test: Create trust anchors from isctest.kasp.Key

Add isctest.kasp.Key.into_ta() method which convert the key into DS /
DNSKEY trust anchor for BIND config. Add a shared template
trusted.conf.j2 which can be linked to in tests to create the trust
anchor configuration from trust anchor data returned from bootstrap()
function.

This is basically a python replacement for the keyfile_to_static_ds (and
friends) from the conf.sh shell framework.

Backport of MR !11201

Merge branch 'backport-nicki/pytest-add-trust-anchor-template-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11294