feat: VectorStoreEntry should have a reference to a Source object instead of bytes

[ludwiktrammer]

1. Replace image_bytes in VectorStoreEntry with image_source (typed as an optional Source object)
2. Make sure that all vector stores properly serialize and deserialize this object when saving/retrieving results (especially Chroma and pgvector)
3. Add a new type of Source - BytesSource, that have the file data directly saved as bytes inside of it.