FBX: support new color attributes, and sRGB/Linear/None colors option (T98721)

FBX exporter code was only supporting "Face Corner - Byte Color"
attribute type, using the deprecated Mesh.vertex_colors API. Update
the to use the new attributes API, and to handle all the attribute
combinations.

An option to pick between sRGB (default, matches current behavior),
Linear (requested in T82847) or None (don't export/import any color
attributes) has been added to the exporter and the importer.

Reviewed By: Bastien Montagne
Differential Revision: https://developer.blender.org/D15942