(WIP) Integrations: Weights and Biases Tracer Callback

[tssweeney]

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In this PR, we update the callback handler in order to leverage the Tracer abstraction. This dramatically simplifies the integration code itself and leverages new LangChain media types exposed by W&B. As a result, the visualizations for such types can be customized.

Depends on: https://github.com/wandb/wandb/pull/5288 being landed and deployed in W&B library.

[yogeshg]

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