Implement stream and astream for RunnableLambda (#14794)
**Description:** Implement stream and astream methods for RunnableLambda
to make streaming work for functions returning Runnable
- **Issue:** https://github.com/langchain-ai/langchain/issues/11998
- **Dependencies:** No new dependencies
- **Twitter handle:** https://twitter.com/qtangs
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Co-authored-by: Nuno Campos <nuno@langchain.dev>
