Add a new DPCTLEvent_GetWaitList function

[vlad-perevezentsev]

This PR adds the 'DPCTLEvent_GetWaitList' function which returns the list of events that this event waits for in the dependence graph.

[coveralls]

Coverage decreased (-0.1%) to 65.046% when pulling b5374bb on vlad-perevezentsev:event_getwaitlist into 893f1f6 on IntelPython:master.

[vlad-perevezentsev]

@oleksandr-pavlyk I added a Sycl test for function.
It works on SYCL but test fails with DPCTLSyclEvent.

/localdisk/work/vperevez/dp_dparray/dpctl_vlad/dpctl/dpctl-capi/tests/test_sycl_event_interface.cpp:158: Failure
Value of: DPCTLEventVector_Size(EVRef) > 0
  Actual: false
Expected: true

Dependencies passed via ' sycl:: handler::depends_on` are not included in the graph, but are tracked (Compiler LLVM team).
This is a bug. https://jira.devtools.intel.com/browse/CMPLRLLVM-30013

[oleksandr-pavlyk]

I have applied the requested changes and resolved conflicts from earlier merges