core[minor]: add upsert, streaming_upsert, aupsert, astreaming_upsert methods to the VectorStore abstraction (#23774)

This PR rolls out part of the new proposed interface for vectorstores
(https://github.com/langchain-ai/langchain/pull/23544) to existing store
implementations.

The PR makes the following changes:

1. Adds standard upsert, streaming_upsert, aupsert, astreaming_upsert
methods to the vectorstore.
2. Updates `add_texts` and `aadd_texts` to be non required with a
default implementation that delegates to `upsert` and `aupsert` if those
have been implemented. The original `add_texts` and `aadd_texts` methods
are problematic as they spread object specific information across
document and **kwargs. (e.g., ids are not a part of the document)
3. Adds a default implementation to `add_documents` and `aadd_documents`
that delegates to `upsert` and `aupsert` respectively.
4. Adds standard unit tests to verify that a given vectorstore
implements a correct read/write API.

A downside of this implementation is that it creates `upsert` with a
very similar signature to `add_documents`.
The reason for introducing `upsert` is to:
* Remove any ambiguities about what information is allowed in `kwargs`.
Specifically kwargs should only be used for information common to all
indexed data. (e.g., indexing timeout).
*Allow inheriting from an anticipated generalized interface for indexing
that will allow indexing `BaseMedia` (i.e., allow making a vectorstore
for images/audio etc.)
 
`add_documents` can be deprecated in the future in favor of `upsert` to
make sure that users have a single correct way of indexing content.

---------

Co-authored-by: ccurme <chester.curme@gmail.com>

libs/community/langchain_community/vectorstores/milvus.py

@@ -1053,7 +1053,7 @@ class Milvus(VectorStore):
         pks = [item.get(self._primary_field) for item in query_result]
         return pks
 
-    def upsert(
+    def upsert(  # type: ignore[override]
         self,
         ids: Optional[List[str]] = None,
         documents: List[Document] | None = None,