Implement condensed context conversation chat engine (#8949)

CHANGELOG.md

docs/api_reference/query/chat_engines.rst

.../query/chat_engines/condense_plus_context_chat_engine.rst

...mples/chat_engine/chat_engine_condense_plus_context.ipynb

llama_index/chat_engine/__init__.py

llama_index/chat_engine/condense_plus_context.py

llama_index/chat_engine/types.py

llama_index/indices/base.py

tests/chat_engine/test_condense_plus_context.py