[MEDI] Allow VectorStoreWriter to be given an already-created MEVD VectorStoreCollection

[roji]

VectorStoreWriter is currently only instantiable with a VectorStore (a "database"), and internally creates the VectorStoreCollection (the thing representing a "table") when it's handed the first chunk (among other things, based on the contents of that chunk, see #6968).

We should also allow users to construct a VectorStoreWriter over an already-created VectorStoreCollection; the main use-case here is if users want to configure the collection in some specific way.

Note that since VectorStoreCollection closes over the model/schema, the user would have to build it with the correct model; failing to do so would cause an error, the first time they attempt to ingest a chunk. This makes it a relatively advanced API.

Not a blocking API for GA - can be added later.