Add signature_request_reassigned as a callback event to be handled

[latoyazamill]

signature_request_reassigned is not listed as an event here https://github.com/hellosign/hellosign-java-sdk/blob/master/src/main/java/com/hellosign/sdk/resource/support/types/EventType.java

Hence not being able to get validated when customers are attempting to handle callbacks for this particular request. As detailed in this example: https://faq.hellosign.com/hc/en-us/articles/115002045507-Example-Java-callback-handler