Feature Request: Langfuse Integration

Feature Request:
As the title suggest I wanted the langfuse support inside the repo to see exactly what was being passed and returned in a multiple document run and I went ahead and added the support in a fork of the repository.

I think I went the wrong way around, I needed the feature support quick enough so I went ahead and added it for our personal use case, but came this morning and thought other people could possibly benefit from this as well since I found a few issues with people questioning too much token usage with deepseek (#303), they can observe with this observability tool where their tokens are being used.

What to do now:

I created a PR (tests, pre-commit, autoformat) all passing but I feel like the change is more than 1000 lines so it might get rejected, if any rework required let me know and I can ensure it merges according to google coding standards, otherwise for other out there they can access this fork here: https://github.com/UYousafzai/langextract/tree/langfuse-integration

branch: langfuse-integration