Extend TokenUsage to Support Custom Fields for Enhanced Usability

[nallwhy]

Currently, the TokenUsage module in LangChain only supports input and output fields, limiting its flexibility for use cases requiring additional metadata, such as cached_tokens. By allowing extensions to include a raw field or other custom fields, users could store and retrieve more comprehensive information tailored to their specific needs.

%LangChain.TokenUsage{
  input: 19,
  output: 10,
  raw: %{
    "completion_tokens" => 10,
    "completion_tokens_details" => %{
      "accepted_prediction_tokens" => 0,
      "reasoning_tokens" => 0,
      "rejected_prediction_tokens" => 0
    },
    "prompt_tokens" => 19,
    "prompt_tokens_details" => %{"cached_tokens" => 0},
    "total_tokens" => 29
  }
}

[brainlid]

Closed by PR #236