dpctl.SyclPlatform should provide get_devices method

[oleksandr-pavlyk]

This would correspond to sycl::platform::get_devices(deviceType), see table 16 in 4.6.2.1 of SYCL 2020 standard

https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#table.members.platform

[oleksandr-pavlyk]

It should also support the platform-specific default context, provided by oneAPI extension.

[oleksandr-pavlyk]

It should also support the platform-specific default context, provided by oneAPI extension.

This was implemented in #827