rec: Support exporting more record types via protobuf

[rgacogne]

Short description

Several users have reported the need to export the content of of more record types found in responses. This pull request adds support for optionally exporting the content of MX, NS, PTR, SPF, SRV and TXT record types, in addition to the existing A, AAAA and CNAME. The implementation is limited to those types because it special cases them.
An other option would be to implement all xfr*() methods to be able to get the "wire version" of all types. If we decided to go that road, I think it might make sense to extract the xfr*()methods from DNSPacketWriter into a DNSRecordWriter implementing all xfr*() methods, that would be called by DNSPacketWriter but could also be used from the protobuf logger.

This PR is based on #6698 and will need a rebase after it has been merged.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[chbruyand]

I'd vote in favor of the DNSRecordWriter.

As far as I understand, your current implementation is not really currently limited to exporting the types listed in your comment as you check known types against QType::names.

[rgacogne]

As far as I understand, your current implementation is not really currently limited to exporting the types listed in your comment as you check known types against QType::names.

It is, because RecProtoBufMessage::addRR() only knows how to deal with those types.

[rgacogne]

I'd vote in favor of the DNSRecordWriter.

It would be nicer, yes, but would also be quite more work. My current feeling is to merge this pull request and see if the demand for exporting other types really exists before considering the DNSRecordWriter option.

[rgacogne]

Rebased since #6698 has been merged in the meantime.