Add QueryUnderstandingAgent LlamaPack (#11558)

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

...ks/llama-index-packs-query-understanding-agent/.gitignore

...x-packs/llama-index-packs-query-understanding-agent/BUILD

...acks/llama-index-packs-query-understanding-agent/Makefile

...cks/llama-index-packs-query-understanding-agent/README.md

...erstanding-agent/examples/query_understanding_agent.ipynb

...g-agent/llama_index/packs/query_understanding_agent/BUILD

...t/llama_index/packs/query_understanding_agent/__init__.py

...agent/llama_index/packs/query_understanding_agent/base.py

...agent/llama_index/packs/query_understanding_agent/step.py

...lama-index-packs-query-understanding-agent/pyproject.toml

...ma-index-packs-query-understanding-agent/requirements.txt

...s/llama-index-packs-query-understanding-agent/tests/BUILD

...a-index-packs-query-understanding-agent/tests/__init__.py

...nding-agent/tests/test_packs_query_understanding_agent.py