Allow state management of tools

[TomeHirata]

This PR allows tools to maintain states during the lifespan of each ReAct.forward without affecting other executions by making the copies of tool objects. Users can create a tool class that mutates instance variables.

class Counter():
    def __init__(self):
        self.count = 0

    def __call__(self, i: int):
        self.count += i
        return self.count

counter = Counter()

react = dspy.ReAct("max -> sum:int", tools=[counter])

[chenmoneygithub]

One minor comment

[chenmoneygithub]

we may want to wrap this in a try-except block and fall back to shallow copy in cases that deepcopy fails