Be able to inject trace_context into LangchainCallbackHandler

[fhoering]

[!IMPORTANT] Add trace_context parameter to LangchainCallbackHandler to enable trace context injection for observations.

• Behavior:
  • Add trace_context parameter to LangchainCallbackHandler constructor for injecting trace context.
  • Use trace_context in on_chain_start() to start observations with context when obs is an instance of Langfuse.
• Imports:
  • Import TraceContext from langfuse.types.

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[hassiebp]

Thanks a lot for your contribution @fhoering - will push this over the line in https://github.com/langfuse/langfuse-python/pull/1458