`when` for output artifacts

# Summary

Ability to optionally output artifacts, just like `dag` or `steps` `when`.

# Use Cases

A validation pipeline running on `main` needs to upload artifacts whereas other branches do not.

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**Message from the maintainers**:

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Compare how conditional `when` works for `dag` and `steps` with the output-artifact behavior described here. Trace the workflow artifact handling entry points and define done as allowing artifacts to be uploaded conditionally, such as only on `main`, without changing unconditional artifact behavior.

Written by the indexing model from the issue text.