Add langchain tool support

[TomeHirata]

Similar to how we support MCP, we support Langchain Tools by providing a conversion method from Langchain Tools to DSPy tool. I will follow up with a tutorial for the langchain tool usage with ReAct.

import dspy
from langchain_community.tools.yahoo_finance_news import YahooFinanceNewsTool
lm = dspy.LM(model='openai/gpt-4o-mini', max_tokens=10000, temperature = 1.0)
dspy.configure(lm=lm, cache=False)
react = dspy.ReAct("query -> result", tools=[dspy.Tool.from_langchain(YahooFinanceNewsTool())])
res = await react.acall(query="Any interesting news happen today?")
res.result

-> "Interesting news today includes discussions about Apple's competitive challenges against its peers in the tech sector and Elon Musk's announcement regarding the end of his role with DOGE, hinting at a shift in focus back to Tesla and its operations."

[okhat]

LGTM we should merge this right after we finish releasing 2.6.25 correctly. Once that's out, anyone can merge. Don't wait for me.