core[minor]: Allow injection of AsyncLocalStorage

[jacoblee93]

AsyncLocalStorage a very useful API that is available in many environments but not all. We want to allow projects like LangGraph to take advantage of its convenience at the cost of supporting only a subset of environments.

The idea would be (before instantiating any RunnableLambda):

import { AsyncLocalStorageProviderSingleton } from "@langchain/core/singletons";
import { AsyncLocalStorage } from "node:async_hooks";

AsyncLocalStorageProviderSingleton.setClass(AsyncLocalStorage);

This PR also allows nested RunnableLambdas to automatically fetch config from the next level up, enabling more convenient tracing.

@nfcampos @bracesproul @dqbd

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[jacoblee93]

I actually think it might be better to instantiate a singleton instance instead of a class - don't think there are any downsides to sharing an instance?

Going to change to that to remove janky race condition around instantiating a global variable in the constructor.

[bracesproul]

Not sure Mock is the right word to use here? To me if I see this I'll assume it should be used in tests, and not as the default value.

[jacoblee93]

It is a mock though because it's not doing anything? Any better names?

[jacoblee93]

@nfcampos how's this look?