Add [a]delete_by_metadata and [a]update_metadata methods to vector store

[hemidactylus]

This PR adds four methods to the vector store, the a/sync versions of:

• update metadata (of several documents at once). This has the semantic of updating a dictionary, i.e. old keys are not lost and overlapping keys are overridden;
• delete (a number of documents) by a metadata filter.

These methods are needed by the operation of "promoting" a vector store to a graph vector store.

[hemidactylus]

@epinzur FYI