Define scope of designation for domains

[tfpauly]

From @enygren:

We need to crisply define the scope of "designation" for DRRs
(designated recursive resolver) as well as what answers
from them can be trusted. Along with this, we should
be crystal clear about which SVCB record designations
for DRRs are designating for which domains.
For example, if I have:

  www.example.com  CNAME  svc.example.net.
  svc.example.net	HTTPSSVC  1 svc2.example.net. ( dohuri=https://doh.example.org/query [...] )

when which domains does that dohuri correspond to?
And which lookups can and should then go to which resolvers?
For example, does that dohuri apply to www.example.com,
svc.example.net, or svc2.example.net, or some mixture of those?

It would be really nice if example.net can specify a DRR
that gets used for svc2.example.net and/or all of example.net.
One possibility would be that HTTPSSVC records don't themselves
include dohuri or odnskeys but instead reference others.
For example, the parameter might be "drrdomain=example.net,example.com"
which would then prompt the client to do SVCB lookups for _drr.example.com
and _drr.example.net.