chg: dev: move handle to keystores from the view to zonemgr

This is a follow-up of !10895 where the keystore pointer was removed
from the zone (as not specific to the zone) and moved to the view. But
in order to avoid adding extra lifecycle dependencies from the zone to
the view, the keystore pointer is now moved to the zonemgr, which also
makes more sense as this is a global settings, and zonemgr wraps a bunch
of other global settings to be accessibles from the zones.

Because the zonemgr lifecycle is the same of the keystores (which are
both depending on named_g_server) this should be a safe change.

Merge branch 'colin/keystores-zonemgr' into 'main'

See merge request isc-projects/bind9!10901