Support priority definition for userTasks

[Skaiir]

What

We should support a simple priority field as part of feature parity.

<userTask id="" name="">
  <bpmn:extensionElements>
    <zeebe:priorityDefinition priority="15" />
  </bpmn:extensionElements>
</userTask> 

How

The field should be an extension element as the bpmn schema only allows integer definitions of a priority. It should also parse as a string for the same reasons.

Related to https://github.com/camunda/product-hub/issues/217
Related to camunda/camunda-modeler#4450

[barmac]

Released via v1.5.0

[barmac]

Reopening due to https://github.com/camunda/product-hub/issues/217#issuecomment-2275277482

I will update the top comment too.