feat: adds langfuse callback handler (#11324)

docs/examples/callbacks/LangfuseCallbackHandler.ipynb

docs/module_guides/observability/observability.md

llama-index-cli/llama_index/cli/upgrade/mappings.json

...-index-core/llama_index/core/callbacks/global_handlers.py

llama-index-core/llama_index/core/command_line/mappings.json

...tegrations/callbacks/llama-index-callbacks-langfuse/BUILD

...rations/callbacks/llama-index-callbacks-langfuse/Makefile

...ations/callbacks/llama-index-callbacks-langfuse/README.md

...x-callbacks-langfuse/llama_index/callbacks/langfuse/BUILD

...backs-langfuse/llama_index/callbacks/langfuse/__init__.py

...callbacks-langfuse/llama_index/callbacks/langfuse/base.py

...s/callbacks/llama-index-callbacks-langfuse/pyproject.toml