luci-mod-network: add DNR (RFC9463) support

luci-mod-network: add DNR (RFC9463) support

odhcpd has had support for Discovery of Network-designated Resolvers
(DNR, RFC9463), which allows allows devices on the network to discover
encrypted DNS resolvers, since December 2024 [1].

This patch adds a basic luci interface to configure DNR records. odhcp
actually supports DNR records via DHCPv4, DHCPv6 and RA, but since it's
typically only used for the latter two, stashing the option next to IPv6
DNS server options seems reasonable.

Patch v2: long help string broken into multiple strings, "Format: ..."
changed to "Syntax: ...".

Patch v3: add a note on the _lifetime=<seconds> parameter, add a RA
<abbr> stanza.

Patch v4: change title to "Announce encrypted DNS servers", change IPv6
example address to the IANA documentation address space, add a blurb
about IPv4 address support (only when odhcpd is used for DHCPv4).

[1] https://github.com/openwrt/odhcpd/commit/6d342cc03bf72f27d0838942d009bf1c5df4f954

Signed-off-by: David Härdeman <[email protected]>