[gold] baseline_server.go: Add /json/v2/expectations route.
[gold] baseline_server.go: Add /json/v2/expectations route.

Bug: skia:10522
Change-Id: If0abd50c341a5249050510d505281a007bba35a8
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/327323
Reviewed-by: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>
Commit-Queue: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot