Add validation for JSON objects (Go maps).
The map keys are called "properties."
As with arrays, annotations are needed to support the
unevaluatedProperties keyword.
Change-Id: Ia01f74300f4bcdbce94065572d83c46dfaa53ae7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668256
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
