Add more rrtypes to dns-record

[emilhf]

While this does make this object slightly more similar to the passive-dns object, I would argue that the purpose is different: The passive DNS object models aggregated DNS intelligence, whereas the dns-record object deals with observables noted by the analyst. There also might be other record types that should be added, but this should deal with the most common ones.

Common use cases for these new record types include:

• Tracking emails from all valid SPF senders used by a spammer (TXT or SPF)
• Correlating validation tokens (TXT) across multiple fronting domains
• Tracking load balanced infrastructure exposed via SRV records, i.e. cloud stuff.
• Modelling CNAME chains

[adulau]

Thank you very much! It makes sense.