Add CEP for optional dependencies in repodata.json

[wolfv]

Trying to trigger some discussion even if this is far from finished :)

[kenodegard]

Here's some history on this very idea! https://github.com/conda/conda/issues/7502

[kenodegard]

Related: https://github.com/conda/conda/issues/11053

[h-vetinari]

Is there a particular reason for sorting the option before the main package?

Related: https://github.com/conda/conda/issues/11053

Thanks for linking this @kenodegard - I think it would be awesome to have syntax like package[*args, **kwargs], which can be used both for optional package extensions à la pip (*args) as well as build variants (**kwargs; e.g. gpu=None, gpu=cuda)