odhcpd: add DNR (RFC 9463) support

odhcpd: add DNR (RFC 9463) support

Discovery of Network-designated Resolvers (DNR) allows devices on the network
to discover encrypted DNS resolvers, which has so far required either manual
configuration or other approaches (like systemd-resolved's "opportunistic"
mode).

To enable DNR, a new uci parameter has been added, which needs to contain at
the very least, the priority (1-65535, lower = higher priority) and the server
hostname (Authentication Domain Name, ADN, to use the wording of RFC9463):

config dhcp 'lan'
        …
        list dnr '100 foobar.example.com'

Optionally (and preferably), a comma-separated list of IP addresses and
SvcParams can also be specified, like this (line wrapping added):

config dhcp 'lan'
        …
        list dnr '100 resolver1.example.com
                    fda7:ab54:69fb::1,fda7:ab54:69fb::2,10.0.0.1
                    alpn=dot port=853'
        list dnr '200 resolver2.example.com
                    fda7:ab54:69fb::2,10.0.1.1,10.0.1.2
                    alpn=dot port=853'

Client support is on it's way (e.g. in systemd PR #30952 or in the Windows
Insiders program).

Signed-off-by: David Härdeman <david@hardeman.nu>