feat: added mm-rag example and started the response synthesis for it

examples/multimodal/rag.ts

examples/multimodal/retrieve.ts

packages/core/src/Node.ts

packages/core/src/QueryEngine.ts

packages/core/src/index.ts

packages/core/src/indices/BaseIndex.ts

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

packages/core/src/indices/summary/SummaryIndex.ts

packages/core/src/indices/vectorStore/VectorStoreIndex.ts

packages/core/src/llm/LLM.ts

...es/core/src/synthesizers/MultiModalResponseSynthesizer.ts

packages/core/src/synthesizers/ResponseSynthesizer.ts

packages/core/src/synthesizers/builders.ts

packages/core/src/synthesizers/index.ts

packages/core/src/synthesizers/types.ts

packages/core/src/tests/CallbackManager.test.ts