Open KeyProvider for Extension

[flobilosaurus]

Hello, we are using your library in a Java Application to test our system E2E using OIDC. It works like charm but we would be happy if we could use our own (simple) Implementation of a KeyProvider (Part of OAuth2Config). The current KeyProvider is a open class but its functions are not open and cannot be overwritten. Could you please seperate implementation and usage by an interface (IKeyProvider) in OAuth2Config or open up the relevant functions in your KeyProvider class?