support langchain>=0.3 and migrate to use pydantic v2

[michaelchia]

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Problem

Langchain 0.3 has been released since 16 Sep. https://python.langchain.com/docs/versions/v0_3/ This one might involve some migration to support. Changes are mainly regarding migrating stuff to pydantic v2.

Proposed Solution

Will need to move all the pydantic stuff away from langchain.pydantic_v1 and to use pydantic v2 from the base package.