add SVCB record

[miekg]

See https://tools.ietf.org/html/draft-ietf-dnsop-svcb-httpssvc-01. We should add this record as a private RR type for the time being. Wireformat is fairly straight forward, except for the value field, which resembles TXT but not entirely and it can be switched to something else by a previous value in the RR.

I was thinking off:

// SVCB RR. See RFC xxxx (https://tools.ietf.org/html/draft-ietf-dnsop-svcb-httpssvc-01)
type SVCB struct {
	Hdr      RR_Header
	Priority uint16
	Target   string   `dns:"domain-name"`
	Value    []string `dns:"txt"` // this is wrong, if priority == 0 this must be something else entirely
} 

The draft talks about SvcFieldPriority instead of Priority, which I really dislike, hence Priority in the above example. Target is a better name than "SvcDomainName" which is used in the draft - this also mimics the SRV record a bit.

[miekg]

The wiredata is kinda crazy, so I wonder what's up with that.

question asked: https://mailarchive.ietf.org/arch/msg/dnsop/KxThjOYEKbzaUZXtoZ_Hl0yRVyY