Use DI container for setting up userkeymanager

[dmitriim]

Currently we set up userkeymanager directly in auth_plugin_userkey constructor. We also have set_userkey_manager method that is being used in tests only.

All this can be improved by using DI container + hook for setting up userkeymanager.

[MagnusSamuelsson]

It would be fun doing this to learn something about moodle DI.

I think we should remove the $config injection through the constructor, and give user_key_manager the responsibility to collect it by itself using get_config.
And use DI for database and time.