acme: add client.DNS01ChallengeRecord

This change adds implementation for dns-01 challenge responses.

Change-Id: I22b2b304ca4a2caeec2980feff19a6c2bc277d8c
Reviewed-on: https://go-review.googlesource.com/27613
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed