Create an extension point for adding extra password validation functionality

Create a "local" plugin that implements the <pluginname>_check_password_policy() function. The plugin function should accept a single string argument; this should be the password that was entered. The plugin function should output a single string value, this should be an error message if the password is invalid, or a blank string if it is ok. Testing can be done by checking that the password is passed into the function correctly, that the return value is treated as an error message in the same way as existing password check errors. You should also check that the existing password check mechanism continues to work as expected, per its own configuration. The plugin should be called regardless of whether the primary password check is enabled or not, because plugins can be enabled separately. I have also created a plugin of my own that implements this callback. You can find it here: https://github.com/Spudley/moodle-local_hibp . This is a 'real' plugin that implements the "HaveIBeenPwned" API as I mentioned in the original ticket description. Please feel free to use this plugin to test the callback as well.