Description of Changes

Based on #4780.
Expose unstable outbox macro to let remote reducer call from rust module.

TODO: should be merged behing unstable-idc feature flag.

API and ABI breaking changes

Additive change, but unstable.

Expected complexity level and risk

2

If you answered more than a 2, explain what is complex about the PR,
and what other components it interacts with in potentially concerning ways. -->

Testing

Adds a smoketest for e2e testing of outbox feature.