public String unmarshall(JsonUnmarshallerContext unmarshallerContext) throws Exception {
            String stringValue = super.unmarshall(unmarshallerContext);
            return !unmarshallerContext.isInsideResponseHeader()
                   ? stringValue
                   : new String(Base64.decode(stringValue), Charset.forName("utf-8"));
        }
Unmarshaller for fields with JSON values. For headers, JSON values are base-64 encoded and are decoded here. For payloads,
JSON values are treated like normal strings.
aws-java-sdk-core/src/main/java/com/amazonaws/transform/SimpleTypeJsonUnmarshallers.java