[Store] Add a way to store document content along with the embeddings (ie when using Chroma DB)

[dorrogeray]

I would like to be able to store the original document content in the Chroma DB records, but the VectorDocument currently has no way to pass that in, other than the metadata. I feel like this requirement will not be specific just to Chroma DB, but various databases used for embeddings will support storing of the original document content along with the vectors.

Any suggestions on how to approach this holistically? Should the VectorDocument be expanded with optional field like ?string $content = null?

Ref PR for a hacky solution - now I am looking for a clean way to achieve this..