resolved: add new "DNS Delegate" concepts (#34368)

Various long standing issues (at least: #5573 #14159 #20485 #21260
#24532 #32022 #18056) have been asking for a way to delegate DNS
resolution of specific domains to very specific DNS servers.

This PR goes a major step towards that goal by adding a new concept "DNS
Delegate" which allows to configure just that. Basically, this adds a
third kind of DNS scope to resolved's logic: besides the per-link and
global DNS scopes there are now also "delegate" scopes, which can be
created by dropping in a new file /etc/systemd/dns-delegate/*.conf. They
carry DNS= and Domains= lines just like the global setting or what the
per-link configuration can carry.

And they are consulted the same way as link DNS scopes are considered,
following the same routing rules.

This allows to configure these DNS delegates statically via drop-in
files as mentioned, and only adds the most basic functionality. Later on
we might want to extend this:

1. Allow dynamic creation of DNS delegates via IPC with lifecycle bound
to IPC client (usecase: installing a DNS delegate that routes traffic to
some DNS-over-TLS server once basic setup is complete).
2. Allow configuration of protocol details per delegate the same way
this is currently allowed per-link.
3. Instead of strictly using DNS as delegation protocol, support an
alternative varlink based protocol (without retransmission problems and
so on) that systemd-machined and similar can implement.

This PR is not complete yet. Lacks docs and tests. Seems to work fine in
my local tests however.

Fixes: #5573
Fixes: #18056
Fixes: #20485