Add CHAOS TXT api.ftl records

[DL6ER]

What does this implement/fix?

Add CHAOS TXT records to dynamically get the available API ports:

• domain.api.ftl will use the configured web domain, e.g. "https://pi.hole:443/api"
• local.api.ftl will use the hard-coded string localhost instead of the domain, e.g., "https://localhost:443/api"
• api.ftl is an alias/shortcut for domain.api.ftl

Redirected ports and duplicates (e.g. defined for IPv4 and IPv6) are skipped. We include the port also when standard ports are used (i.e., 80 for http and 443 for https) as this simplifies the code. There is also nothing bad about being explicit.

There is no round-robin implemented here - this record will always return the ports in their order of being defined in webserver.port

Example:

$ sudo pihole-FTL --config webserver.port "80,81r,[::]:80r,443s,[::]:443s,445s"
80,81r,[::]:80r,443s,[::]:443s,445s

$ dig CHAOS TXT local.api.ftl +short
"http://localhost:80/api/" "https://localhost:443/api/" "https://localhost:445/api/"

$ dig CHAOS TXT domain.api.ftl +short
"http://pi.hole:80/api/" "https://pi.hole:443/api/" "https://pi.hole:445/api/"

$ dig CHAOS TXT api.ftl +short
"http://pi.hole:80/api/" "https://pi.hole:443/api/" "https://pi.hole:445/api/"

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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