[SYCL] Enable discard_events mode for the Level Zero (#6533)

Teach the Level Zero plugin piEnqueue* functions to accept nullptr instead of a pointer to the output event. In this case event is created internally and is not visible externally.
Introduce RefCountExternal for pi_event which allows to track external references to an event. It allows to do some optimizations if we know that event is not externally visible. These optimizations are going to be implemented in subsequent PRs.
Don't create proxy event for batch if there are no externally visible events in the batch.

Author: againull

sycl/include/sycl/detail/pi.h

@@ -49,9 +49,10 @@
 // NOTE that this results in a changed API for `piProgramGetBuildInfo`.
 // 10.12 Change enum value PI_MEM_ADVICE_UNKNOWN from 0 to 999, and set enum
 // PI_MEM_ADVISE_RESET to 0.
+// 10.13 Added new PI_EXT_ONEAPI_QUEUE_DISCARD_EVENTS queue property.
 
 #define _PI_H_VERSION_MAJOR 10
-#define _PI_H_VERSION_MINOR 12
+#define _PI_H_VERSION_MINOR 13
 
 #define _PI_STRING_HELPER(a) #a
 #define _PI_CONCAT(a, b) _PI_STRING_HELPER(a.b)
@@ -564,6 +565,7 @@ constexpr pi_queue_properties PI_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE = (1 << 0);
 constexpr pi_queue_properties PI_QUEUE_PROFILING_ENABLE = (1 << 1);
 constexpr pi_queue_properties PI_QUEUE_ON_DEVICE = (1 << 2);
 constexpr pi_queue_properties PI_QUEUE_ON_DEVICE_DEFAULT = (1 << 3);
+constexpr pi_queue_properties PI_EXT_ONEAPI_QUEUE_DISCARD_EVENTS = (1 << 4);
 
 using pi_result = _pi_result;
 using pi_platform_info = _pi_platform_info;