Design logging for debusine client

This is a spinoff from !359 (comment 444199)

debusine client is likely to become a swiss army knife and debug tool, and it looks quite desirable to be able to debug specific subsystems.

A proposal is to use something like:

parser.add_argument('--debug', nargs='?', type=str, const='all', default=None)

And then do things like --debug=http, --debug=http,parser,auth, or just --debug to activate debugging on all subsystems.

It's also plausible that logging might not always go to stderr, so instead of adding StreamHandlers where debugging is needed the CLI would need to set the debug level to the root logger, and turn off/on debugging for the various subsystems