[SYCL][Graph] Add spec wording for dynamic events

- Adds dynamic events which are updatable between graph executions
- Removes limitations on depending on SYCL events from outside a graph
- depends_on property can now take events/dynamic events
- Allow barriers in the explicit API
- Extend barriers to support dynamic events
- Added get_event() to get event for node execution in a graph
- Add a new graph property which requires that an execution event is available
- Add new examples to the usage guide for dynamic events and command_graph::get_event()