feat: add MetadataReplacementPostProcessor

packages/core/src/ChatEngine.ts

packages/core/src/QueryEngine.ts

packages/core/src/indices/index.ts

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

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

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

...re/src/postprocessors/MetadataReplacementPostProcessor.ts

packages/core/src/postprocessors/SimilarityPostprocessor.ts

packages/core/src/postprocessors/index.ts

packages/core/src/postprocessors/types.ts

...s/postprocessors/MetadataReplacementPostProcessor.test.ts