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.
