Add DSYNC rdatatype support

[peterthomassen]

The RRtype is described here: https://datatracker.ietf.org/doc/draft-ietf-dnsop-generalized-notify/

As the draft has passed WG Last Call and IETF Last Call, and IESG evaluation, no more changes are expected before this becomes an RFC (number not yet known).

[peterthomassen]

According to the spec, compression in the target field is disallowed. _to_wire() just passes compress through from the caller. Do we need to explicitly turn it off?

Note: It's done here as for SVCB, for example, where the spec also does not allow it. I'm not sure if any callers using compress are actually expected.

[rthalley]

You should pass None as the compress parameter of the to_wire() method. I'm confused by your comment about SVCB though, as it does not pass the compress parameter through, but passes None as it should.

[peterthomassen]

I'm confused by your comment about SVCB though

Me too, I misread its code! ;-) Pushed a fix.

[rthalley]

I did a first review pass, and noticed some issues with the "scheme" handling. The draft says that if the scheme has a mnemonic assigned you use it, otherwise you use an unsigned decimal integer. The code in this PR will fail in from_text() if an unsigned decimal integer is given, and fail in to_text() if the rdata's scheme is not known. When fixing this, the from_text() code needs to do the right logic for integer conversion from masterfile, as show in the tokenizer's get_int() method. We could either extract this code to a utility library, or you could unget() the token and call get_int().