Add "Astra DB" vector store integration (#12966)

# Astra DB Vector store integration

- **Description:** This PR adds a `VectorStore` implementation for
DataStax Astra DB using its HTTP API
  - **Issue:** (no related issue)
- **Dependencies:** A new required dependency is `astrapy` (`>=0.5.3`)
which was added to pyptoject.toml, optional, as per guidelines
- **Tag maintainer:** I recently mentioned to @baskaryan this
integration was coming
  - **Twitter handle:** `@rsprrs` if you want to mention me

This PR introduces the `AstraDB` vector store class, extensive
integration test coverage, a reworking of the documentation which
conflates Cassandra and Astra DB on a single "provider" page and a new,
completely reworked vector-store example notebook (common to the
Cassandra store, since parts of the flow is shared by the two APIs). I
also took care in ensuring docs (and redirects therein) are behaving
correctly.

All style, linting, typechecks and tests pass as far as the `AstraDB`
integration is concerned.

I could build the documentation and check it all right (but ran into
trouble with the `api_docs_build` makefile target which I could not
verify: `Error: Unable to import module
'plan_and_execute.agent_executor' with error: No module named
'langchain_experimental'` was the first of many similar errors)

Thank you for a review!
Stefano

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

templates/cassandra-synonym-caching/README.md

@@ -1,7 +1,7 @@
 
 # cassandra-synonym-caching
 
-This template provides a simple chain template showcasing the usage of LLM Caching backed by Astra DB / Apache Cassandra®.
+This template provides a simple chain template showcasing the usage of LLM Caching backed by Apache Cassandra® or Astra DB through CQL.
 
 ## Environment Setup