Adopt attrs converters, rename ServerConfig.type to api_type, fix Location defaults

- Replace manual __init__ methods with attrs converters/factories across
  all model classes, using _flexible_init decorator to handle unknown API keys.
- Rename ServerConfig.type to api_type to avoid shadowing the builtin.
  Update all references in client.py, auth/factory.py, and tests.
- Fix Location.__init__ bug where field() objects were used as parameter
  defaults instead of None (now fixed by removing manual __init__).

Author: iMicknl