[SYCL] Implement sycl_khr_queue_flush

[HPS-1]

Implement khr_flush() as per spec https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:khr-queue-flush, which essentially issues all command in the queue to the device without waiting for them to complete. It seems that there's an existing UR function urQueueFlush() that achieves the "flushing" functionality, so I'm mostly reusing its code (in a new UR func urKhrFlush(), and I'm not directly using it due to the issue stated below).

However there's an issue that, for the OpenCL backend, urQueueFlush() is calling clFinish() (wait for commands to complete spec here) instead of clFlush() (doesn't wait for commands to complete spec here),

despite it clearly says its analogue is clFlush() in unified-runtime/source/loader/ur_libapi.cpp:

So I changed it in urKhrFlush() to call clFlush() instead. But not sure if it's a glitch or intended to use clFinish() for urQueueFlush(), can some reviewer from runtime kindly checks this out? If it's a glitch then I think we can just fix it and directly use urQueueFlush() for khr_flush()?