Added support for CNAME record add/remove

[marank]

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What does this PR aim to accomplish?:
A detailed description, screenshots (if necessary), as well as links to any relevant GitHub issues
I added support for adding/removing CNAME records via pihole -a addcustomcname and pihole -a removecustomcname.

How does this PR accomplish the above?:
A detailed description (such as a changelog) and screenshots (if necessary) of the implemented fix
All CNAME records get written to /etc/dnsmasq.d/05-pihole-custom-cname.conf. This file is included by dnsmasq/pihole-FTL by default.

What documentation changes (if any) are needed to support this PR?:
A detailed list of any necessary changes

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[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-5-1-released/35577/1