[SYCL] Add Unified Runtime Plugin

[smaslov-intel]

Add new pi_unified_plugin that implements only piPlatformsGet (via Unified Runtime zerPlatformGet)
The next step is to add a new backend to SYCL and actually start calling to pi_unified_plugin

Signed-off-by: Sergey V Maslov sergey.v.maslov@intel.com

[smaslov-intel]

This is the only tricky change I would say. Basically some functions in "Unified Runtime", those that allocate new handles, act as a factory. The Level-Zero PI Plugin handle inherits from the Unified Runtime extending it with more functionality (still gradually all moving to Unified Runtime). Since this is a short-living solution think these "bindings" appeared to be good enough.

[steffenlarsen]

Apart from a couple missing newlines at the end of some files, I am okay with this.

[smaslov-intel]

@pvchupin , @againull : please review/merge

[againull]

As we discussed offline, the only concern is that we removed the ability to build plugin using provided headers and loader. Other changes look good to me.

[smaslov-intel]

As we discussed offline, the only concern is that we removed the ability to build plugin using provided headers and loader.

I retained the ability to use custom provided L0 and UR: d5c468e

Hand verified with:
$ python3 ~/sycl/pub/buildbot/configure.py -t Debug --l0-headers=/path/to/include/level_zero --l0-loader=/path/to/libze_loader.so

[smaslov-intel]

The testing is gated by #7730

[pvchupin]

@smaslov-intel, there are post-commit issues on windows and clang build: https://github.com/intel/llvm/actions/runs/3711245053

[smaslov-intel]

@smaslov-intel, there are post-commit issues on windows and clang build: https://github.com/intel/llvm/actions/runs/3711245053

#7827