This patch adds a channel display option in the Image Editor that allows to view a matte for any object based on Cryptomatte render passes (therefore, it depends on D3538 of course).
It's far from finished - for example, you can't (yet) select multiple objects and it doesn't work for the material/asset mode.
I'm publishing it now because I'm looking for feedback: Is this useful? Is it in the right place? Do we even want to add something like this?