fix: honor required fields in jackson @JsonProperty annotations #895

Job	Run time
Build Java Client JDK8 (samples/client/echo_api/java/resttemplate)	1m 10s
Build Java Client JDK8 (samples/client/echo_api/java/apache-httpclient)	1m 40s
Build Java Client JDK8 (samples/client/echo_api/java/resteasy)	48s
Build Java Client JDK8 (samples/client/echo_api/java/feign-gson)	54s
Build Java Client JDK8 (samples/client/echo_api/java/okhttp-gson)	52s
	5m 24s

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