Better async support

[itay1542]

dspy.asyncify is a good start but it doesn't feel idiomatic. Would be cool if we had an AsyncModule where the forward method is async or alternatively add an aforward method to dspy.Module. The reason this would be useful is

1. The user will be able to control parallelization inside their Module's forward and
2. This will enable async tool calls with dspy.ReAct for example
3. Better integration with async frameworks such as FastAPI etc..

At the moment, using dspy.asyncify on a Module doesn't allow easy async programming inside the forward method because you can't use the await keyword, you have to resort to interacting with the active asyncio loop etc..