Collect unknown keywords as annotations

And try to avoid memory exhaustion attacks, which were possible
even with out this change but potentially more likely to happen
by accident with it (e.g. unrecognized applicator with a very
large subschema).

Author: handrews

jsonschema-core.xml

@@ -342,7 +342,7 @@
                     </t>
                     <t>
                         A JSON Schema MAY contain properties which are not schema keywords.
-                        Unknown keywords SHOULD be ignored.
+                        Unknown keywords SHOULD be treated as annotations.
                     </t>
                     <t>
                         An empty schema is a JSON Schema with no properties, or only unknown
@@ -3036,6 +3036,11 @@ https://example.com/schemas/common#/$defs/count/minimum
                 system resources.
                 Validators MUST NOT fall into an infinite loop.
             </t>
+            <t>
+                A malicious party could cause an implementation to repeatedly collect a copy
+                of a very large value as an annotation.  Implementations SHOULD guard against
+                excessive consumption of system resources in such a scenario.
+            </t>
             <t>
                 Servers MUST ensure that malicious parties can't change the functionality of
                 existing schemas by uploading a schema with a pre-existing or very similar "$id".