Allow client_debug_name to be specified in config.json


The purpose of this commit is to specify a debug name for the client in the config file, which will be displayed in the prompt.

It might theoretically be useful for power users who frequently interact with multiple clients over CLI to add the client name to the prompt (to keep them from entering a command in the wrong client), but such users probably already use differently named wallets for the purpose if they care about that.

The reason I'm even spending time on this is that I'm working on improving our testing framework, and being able to use a string identifier to name multiple client objects in a single process will be quite useful for testing framework purposes.