Add chat-identifiers endpoint

[nenb]

This PR adds an endpoint that returns a list of mappings from chat UUID to chat name for all chats that a user has created. This is useful when a user would like to perform actions on specific historical chats.

Currently, the only way to get this information is to make a request for all previous chat information (includes messages, sources etc). This can be impractical when building UIs for users with a large amount of previous chats.