jsonschema: validate object keywords

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>

internal/jsonschema/annotations.go

@@ -14,6 +14,7 @@ type annotations struct {
 	allItems            bool            // all items were evaluated
 	endIndex            int             // 1+largest index evaluated by prefixItems
 	evaluatedIndexes    map[int]bool    // set of indexes evaluated by contains
+	allProperties       bool            // all properties were evaluated
 	evaluatedProperties map[string]bool // set of properties evaluated by various keywords
 }
 
@@ -58,6 +59,9 @@ func (a *annotations) merge(b *annotations) {
 		a.endIndex = b.endIndex
 	}
 	a.evaluatedIndexes = merge(a.evaluatedIndexes, b.evaluatedIndexes)
+	if b.allProperties {
+		a.allProperties = true
+	}
 	a.evaluatedProperties = merge(a.evaluatedProperties, b.evaluatedProperties)
 }