Use parameter object for retrieve function of Retriever (#616)

examples/multimodal/retrieve.ts

packages/core/src/Retriever.ts

packages/core/src/cloud/LlamaCloudIndex.ts

packages/core/src/cloud/LlamaCloudRetriever.ts

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

packages/core/src/engines/query/RetrieverQueryEngine.ts

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

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

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

packages/core/src/objects/base.ts