Add support for editing metadata for existing chunks

[deshraj]

🚀 The feature

Currently, if you ingest some data and want to attach some metadata after indexing the chunks, it is not easy to do that and a developer would have to update the metadata manually.

Instead, we want to be able to edit the metadata (add new key, edit existing key, delete key) from vector store with a simple interface.

Motivation, pitch

Motivation is to make sure that the user doesn't have to index the data twice if they only want to update the metadata for the indexed data.