Support conda 4.4 optional/constrained dependencies

[kalefranz]

With conda/conda#4982, conda is ready to support constrained dependencies, and now we need to add the capability into meta.yaml.

Under the requirements section in meta.yaml, we need to add a run_constrained key that takes a list of specs, just like build and run. The result should be an additional constrains key in index.json, parallel to depends that comes from the run key in meta.yaml.

[github-actions]

Hi there, thank you for your contribution!

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Thanks!