Provide Cython declaration file for usage of SYCL objects

[oleksandr-pavlyk]

Integration of SYCL functions for use in Cython extensions requires author to redeclare SYCL runtime objects.

For example, see

https://github.com/IntelPython/sample-data-parallel-extensions/blob/2cd4743c4364633ad98129636ba0755bf038f0f2/common_ext/cy_kde.pyx#L21-L23

dpctl should provide dpctl/sycl.pxd file which would allow Cython authors to just import the declaration

from dpctl.sycl cimport queue as dpcpp_queue

[oleksandr-pavlyk]

This has been implemented in PR #981