P: Add full support for external editing in Rec2020 (critical for HDR and ideal for print)

The Rec2020 colorspace (aka ITU-R BT.2020) is a critical colorspace for photography and not well supported in Lightroom Classic.

Rec2020 is the preferred colorspace for HDR (and is LRC's working primaries when in HDR mode). Rec2020 is also very ideal for print-related work as it includes many printable colors which fall outside Adobe RGB / P3, and yet avoids the excess of ProPhoto (which includes many imaginary values outside the range of human vision).

You cannot use it via any "edit in" options to open your image in Photoshop, which is especially important for working with RAW Smart Objects. The colorspace options in LRC preferences / external editing are limited to ProPhoto, Adobe RGB, P3, and sRGB.

This would ideally be done as a "JDI", as both LRC and PS support Rec2020, we simply don't have the option in the colorspace dropdown for external editing.

Note that there is limited support now in the "export" pathway (you can select file settings / color space = "rec 2020"). We just need a way to offer the same colorspace options for external editing.