Feature Addition: Allow for selection of custom Media/Chat DBs for non-combined RAG

[rmusser01]

As a user I would like the ability to select a custom or specific/non-standard to my installation database for RAG to be performed against when I am chatting about a topic in a RAG-enabled conversation.

I should be able to select the backing DB, from a file.

This setting may be enabled/modified within the chat interface, but not from the config.txt file.