Merge pull request #18451 from OrestChura:oc/count_non_zero

Merge pull request #18451 from OrestChura:oc/count_non_zero

[G-API]: countNonZero() Standard Kernel Implementation

* Add countNonZero() standard kernel
 - API and documentation provided
 - OCV backend supported
 - accuracy and performance tests provided
 - some refactoring of related documentation done

* Fix GOpaque functionality for OCL Backend
 - test for OCL Opaque usage providied

* countNonZero for GPU
 - OCL Backend implementation for countNonZero() added
 - tests provided

* Addressing comments