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 #5096

[PromoFaux]

Thank you for the contribution, though I think this issue will be addressed in the upcoming (no ETA, just "the future" for now) v6.0

See this comment from @DL6ER here: #1542 (comment)

I'm not going to block this PR if anyone else see's the value in it, but I am hesitant to add this as a feature to the existing web interface only for the functionality to be re-imagined in the ongoing v6 work.