GAPI (fluid): RGB/BGR to gray: optimization (#13162)

* GAPI (fluid): RGB/BGR to Gray: add performance tests

* GAPI (fluid): RGB/BGR to Gray: speedup 8-12x with manual CV_SIMD

* GAPI (fluid): RGB/BGR to Gray: fix compiler warning

* GAPI (fluid): RGB/BGR to Gray: dynamic dispatching to AVX2

* GAPI (fluid): RGB/BGR to Gray: check R/G/B coefficients

* GAPI (fluid): RGB/BGR to Gray: fixed compilation error (caused by change in master)