Thread Local and Async Local history for BaseLM

[TyTodd]

I've ran into this use case a couple times now where I need to get the history of a Predict but feel uneasy about using LM.history because the same LM is being used in multiple coroutines. The devx for getting a thread safe view of history is a little verbose, you either have to use dspy.context with a copy of the global LM. Or run a separate copy of your module if it has its self.lm. In some (rare) cases even this is not enough. I recently ran into a scenario where I needed to use the same module across multiple co-routines. The module had its own self.lm so using dspy.context wasn't a viable solution since it injects the LM into the local dspy.settings.

This short PR adds thread/async-local history to LM via a ContextVar called local_history. Allowing users to view the history from the local thread.