Added specific domain forwarding to other name server

[ImDaBigBoss]

What does this PR aim to accomplish?:

This PR is made so you can create a list of domains that would be forwarded to a different name server.
This is useful if you have run Pi-hole but don't maintain the internal domains. For example, you are on a network that relies on services that use domain names, domain names that are issued internally by a DNS server that you don't control. If you still want access to these local domains, what they point to and their subdomains, you can do so without relying on that DNS server for all requests.
It also applies to VLANs and their hostname resolution (https://discourse.pi-hole.net/t/multiple-subnets-vlans-and-hostname-resolution/31746)

How does this PR accomplish the above?:

Much like the DNS records (A/AAAA records) or the CNAME records, this adds a section under "Local DNS" where you can register domain forwarding.
This creates a new file in the dnsmasq directory and adds server=/my.domain/1.2.3.4 into it.

Link documentation PRs if any are needed to support this PR:

None

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[ImDaBigBoss]

This depends on #2471

[PromoFaux]

Echo comments: pi-hole/web#2471 (comment)

[jpgpi250]

If I understand this correctly, I've been doing this for a very long time, with an additional /etc/dnsmasq.d file, entries like:

server=/b2discourse.b-cdn.net/127.10.10.5#5555
server=/b2discourse.pi-hole.net/127.10.10.5#5555

127.10.10.5#5555 being knot-resolver, also running on my pi, the default upstream resolver is unbound (127.10.10.2#5552)

however....

In the files changed section of this PR, I noticed this code uses RestartDNS to apply the changes, If I'm correct RestartDNS actually does service pihole-FTL restart. This is totally unnecessary.
the dnsmasq man page states:

--servers-file=<file>
A special case of --conf-file which differs in two respects. Firstly, only --server and --rev-server are allowed in the configuration file included. Secondly, the file is re-read and the configuration therein is updated when dnsmasq receives SIGHUP.

this implies adding a configuration line (I added this line in a separate config file - could be added to 01-pihole.conf):

servers-file=/etc/dnsmasq.d/07-pihole-custom-ns.conf

doesn't require RestartDNS but would activate the changes, using RestartDNS reload which actually does:

kill -HUP ${pid}