colorspace: add CIE Y values to pl_hdr_metadata

I've decided that these (luma-based) values are conceptually different enough from the HDR10+ (dynamic) values that we shouldn't mix and match them.

Because the HDR10+ API is already merged and integrated into multiple other codebases, it's too late to touch it, so we have no choice but to add the better API on top. Besides, that lets us choose the HDR data source more flexibly.