DNS options

Add dns section to the free app.  All of the following are readily available features of dnsmasq and likely already used by eero+.

1. dnssec - ideally the dns cache would always use dnssec at a minimum.  Preferably DoH or DoT but those aren't natively supported.
2. DNS hijacking - add firewall/nat rules to redirect traffic to the specified server or the eero's cache if enabled.
3. block/allow domains - allow adding limited number of domains to block or allow.  This is critical for blocking DoH/DoT when hijacking DNS
4. block/allow lists - allow adding urls to host lists to either allow or block.  50 lines of shell script can easily parse, format, dedup and sort these into a conf file for dnsmasq
5. DHCP options - not directly dnsmasq but it would be nice to set custom dhcp options for tftp booting and such.