core[minor],langchain[patch]: Move base indexing interface and logic …

…to core (#20667)

This PR moves the interface and the logic to core.

The following changes to namespaces:


`indexes` -> `indexing`
`indexes._api` -> `indexing.api`


Testing code is intentionally duplicated for now since it's testing
different
implementations of the record manager (in-memory vs. SQL).

Common logic will need to be pulled out into the test client.


A follow up PR will move the SQL based implementation outside of
LangChain.

libs/core/langchain_core/indexing/__init__.py

@@ -0,0 +1,15 @@
+"""Code to help indexing data into a vectorstore.
+
+This package contains helper logic to help deal with indexing data into
+a vectorstore while avoiding duplicated content and over-writing content
+if it's unchanged.
+"""
+from langchain_core.indexing.api import IndexingResult, aindex, index
+from langchain_core.indexing.base import RecordManager
+
+__all__ = [
+    "aindex",
+    "index",
+    "IndexingResult",
+    "RecordManager",
+]