feat[cl-fastapi]: draft for new fastapi structure (supporting engines)

templates/types/streaming/fastapi/README-template.md

templates/types/streaming/fastapi/app/api/routers/chat.py

templates/types/streaming/fastapi/app/context.py

templates/types/streaming/fastapi/app/engine/__init__.py

templates/types/streaming/fastapi/app/engine/constants.py

templates/types/streaming/fastapi/app/engine/context.py

templates/types/streaming/fastapi/app/engine/generate.py

templates/types/streaming/fastapi/app/engine/index.py

templates/types/streaming/fastapi/app/utils/index.py

templates/types/streaming/fastapi/main.py