Add support for retry backoff

[aivinog1]

Since the BPMN API for retry backoff negotiated we are ready to implement it in the Camunda Modeler, for example. Adding support for the retry backoff in this library would be the first step, I suppose.

P.S. I am ready to implement this issue, you can assign this to me :)

[nikku]

Closing this issue. Let's re-open it once we have clarity what the desired editor support looks like. Let's use https://github.com/camunda/camunda-modeler/issues/3419 to define that support.