refactor: update low-level streaming interface (#325)

.changeset/tame-planes-relax.md

apps/docs/docs/modules/low_level/response_synthesizer.md

examples/anthropic.ts

examples/gptllama.ts

examples/jsonExtract.ts

examples/llamadeuce.ts

examples/lowlevel.ts

examples/mistral.ts

examples/ollama.ts

examples/openai.ts

examples/portkey.ts

examples/together-ai.ts

examples/vision.ts

packages/core/src/ChatEngine.ts

packages/core/src/ChatHistory.ts

packages/core/src/QueryEngine.ts

packages/core/src/QuestionGenerator.ts

packages/core/src/Response.ts

packages/core/src/indices/keyword/KeywordTableIndex.ts

...es/core/src/indices/keyword/KeywordTableIndexRetriever.ts