Add support to update variables of a non-completed task based on a addition call to zeebe

[StephenOTT]

There are scenarios where a user task could be open for a long period of time and you need to update the user task copy of the variables so the user can get the most recent snapshot of data.

Likely should be a additional endpoint for the user to call from a user task. Such as /tasks-:taskId/variables/update (or something like that)

Can add caching based on updates could only occur with N period. ?

Also consider if the original capture variables should be kept and a new column for updated variables be added. Or if updates should modify the current variables column.