Currently, when making calls to AstraDB, we include a `User-Agent`
header identifying the API caller. This header is currently set to
"langchain" by default at the moment. This change threads the
`caller_name` parameter throughout the API and appends a suffix
identifying which class created the API client:

* langchain/cache
* langchain/chat_message_history
* langchain/document_loader
* langchain/graph_vectorstore
* langchain/semantic_cache
* langchain/vectorstore

This metadata will be useful for debugging and tracking utilization of
different library features.

NOTE: This change will change the value of User-Agent headers, for
example from `langchain/0.3.6` to `langchain/vectorstore/0.3.6`