dns/dnsmessage: add https svcb dns types

This change implements the proposal to add new DNS message types HTTPS and
SVCB in the golang.org/x/net/dns/dnsmessage package, as described in
golang/go#43790.

The implementation includes:
- New types TypeHTTPS and TypeSVCB.
- SVCBResource and HTTPSResource structs, with HTTPSResource embedding
  SVCBResource.
- SVCParam and SVCParamKey types for handling service parameters.
- pack and unpack methods for the new resource types.
- Integration into the Parser and Builder.
- Comprehensive tests, including for parameter handling logic.

I implemented the SVCB parsing code so that it performs only two
allocations: one for the []SVCParam slice, and one to hold the SVCParam
values. A test was added to demonstrate that.

Fixes golang/go#43790

Change-Id: I60439772fe0e339ae3141bd1dd9829564efe0f2a
GitHub-Last-Rev: 49c2ac0102ba982d97d6ef90285c907135d9c176
GitHub-Pull-Request: golang/net#241
Reviewed-on: https://go-review.googlesource.com/c/net/+/710736
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Vinicius Fortuna <fortuna@google.com>