Openfire’s plugin API is very versatile, and can be used to do almost anything - but to add a User, Auth or Group provider, you’d better not use a plugin.
Plugins run in their own, isolated, classloader. This is done by design, but it’s also what is causing the issue that you’re experiencing.
To create an AuthProvider (or Group-, or UserProvider), it is advisable to create an ordinary library (a jar file), and add that to the Openfire ‘lib’ directory, before starting. This will make available your code on the core classpath of Openfire.
You can find some more details on http://download.igniterealtime.org/openfire/docs/latest/documentation/db-integration-guide.html