Support custom priority on orchestration workitem dispatching

[YunchuWang]

This change introduces customer-defined orchestration dispatch priority
It allows customers to assign a priority value to orchestration instances so that higher-priority work items are dispatched earlier when multiple orchestration runs are competing for execution.

This mechanism affects dispatch ordering, not completion order.
While higher-priority instances are more likely to be picked up and started first, completion time is not guaranteed, as execution duration varies per orchestration and multiple work items are processed concurrently across workers.