Replace dependency on langchain with langchain-core

[navaj0]

langchain has been split into several slim packages, including langchain-core, langchain-<aws, ...>. Users, especially who are using LCEL, don't need to install the whole langchain package in their working environment.

However, the langfuse CallbackHanlder still takes a hard dependency on langchain, instead of langchain-core. See https://github.com/langfuse/langfuse-python/blob/main/langfuse/callback/langchain.py#L7

Modules here could be imported from langchain_core.