feat: implement index selector for LlamaCloud (#200)

.changeset/quick-walls-switch.md

helpers/env-variables.ts

helpers/index.ts

templates/components/engines/python/agent/__init__.py

templates/components/engines/python/chat/__init__.py

templates/components/engines/typescript/agent/chat.ts

templates/components/engines/typescript/chat/chat.ts

...tes/components/llamaindex/typescript/streaming/service.ts

templates/components/vectordbs/python/llamacloud/index.py

templates/components/vectordbs/python/none/index.py

templates/components/vectordbs/typescript/astra/index.ts

templates/components/vectordbs/typescript/chroma/index.ts

...lates/components/vectordbs/typescript/llamacloud/index.ts

templates/components/vectordbs/typescript/milvus/index.ts

templates/components/vectordbs/typescript/mongo/index.ts

templates/components/vectordbs/typescript/none/index.ts

templates/components/vectordbs/typescript/pg/index.ts

templates/components/vectordbs/typescript/pinecone/index.ts

templates/components/vectordbs/typescript/qdrant/index.ts

...reaming/express/src/controllers/chat-config.controller.ts