fix(graph): make handler generation async/effectful

[sgrove]

Summary

Graph codegen is currently expected to be pure, which is too restrictive for some use-cases. This PR makes handler generation async to loosen up that requirement.

We may re-introduce an alternate, pure-function approach alongside this in the future that'll have its own benefits, but for now this unblocks some downstream requirements.

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📊 Benchmark results

Comparing with cfeea77

Package size: 222 MB

⬆️ 0.01% increase vs. cfeea77

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• T (current commit): 222 MB