fix: add support for cross environment usage

[averikitsch]

Due to the use of an async driver, sync and async interfaces can not be mixed with the opposite environment. If the Engine was initialized asynchronously, no self-managed loop is created therefore coroutines can not be scheduled to run synchronously.

• If we try to run on the same loop, the task blocks the thread with the current loop and never completes.
• If the Engine was initialized synchronously, coroutines error since it is not attached to the native loop. LangChain interfaces support both async and sync calls for all methods.

https://docs.google.com/document/d/1Cf73cs5hTV0rCakK7rjZAPLk9RIsw1AS7o7GcDoTQg0

[jackwotherspoon]

@averikitsch do you mind adding a brief PR description going over what this fix: is fixing? :smile: