SPF/TXT multipart handling

[weppos]

This PR introduces the multi-part SPF/TXT handling. The underlying RR types are already defined so that the value is a list, but we currently ignore it and we attempt to parse/split the incoming value.

This is related to https://github.com/dnsimple/dnsimple-business/issues/1765, in particular to issues related to the TXT implementation. The current implementation is designed to prefer the parts over the single value, whenever the JSON contains a valid element with the expected value.

Here's some examples.

1. TXT: Both txt and txts exists, txts is used

dig -p 8053 @127.0.0.1 txt2d.multipart.example TXT

txt2d.multipart.example. 120	IN	TXT	"string c" "string d"