feat: use images in context chat engine (#886)

.changeset/afraid-seahorses-learn.md

examples/multimodal/context.ts

examples/multimodal/rag.ts

packages/core/src/engines/chat/ContextChatEngine.ts

packages/core/src/engines/chat/DefaultContextGenerator.ts

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

packages/core/src/synthesizers/utils.ts