Support authorization code flow in balloon v3

[raffis]

Describe the change

The balloon server v3 ships an idp with authorization code flow only. See https://github.com/gyselroth/balloon/issues/435

Current situation

Currently the desktop client supports:

• resource owner password grant
• external idp's
• basic auth
• two factor auth (using a custom resource owner password flow)

Should

Support in v2 will get removed for all of those methods and only the internal balloon idp will be supported. Any 3rd party auth methods can be configured on the balloon auth service in v3.

Additional context

See auth server: https://github.com/gyselroth/balloon-auth-consent

[pixtron]

Also remove deprecated identifying instances by username (see #230). The deprecated changes are marked with // TODO pixtron - deprecated.