Missing field support in JsonToRow

[JsonToRow](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/JsonToRow.java) supports only strict schema matching, so it expects all the fields that are declared by the schema to be present in the input JSON data. If the field is missing, [it crashes](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/util/RowJsonDeserializer.java#L114-L117). In practice it is useful to default to null is such cases. We should make it configurable in Beam Java SDK at least, and probably in Beam SQL (in Pubsub Json TableProvider) as well.

Imported from Jira [BEAM-7624](https://issues.apache.org/jira/browse/BEAM-7624). Original Jira may contain additional context.
Reported by: kedin.