Fix event streams in the orchestrator implementation (#2673)

## Motivation and Context
This PR gets event streams working in the client orchestrator
implementation, and depends on #2671.

The orchestrator's `TypeErasedBox` enforces a `Send + Sync` requirement
on inputs and outputs. For the most part, this isn't an issue since
almost all generated inputs/outputs are `Send + Sync`, but it turns out
the `EventStreamSender` wasn't `Sync` due to an omission of the `Sync`
bound. Thus, this PR is a breaking change, as it adds a `Sync`
requirement for anyone who passes a stream to an event stream operation.

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