Oauth2: add ability to decorate AuthenticationProvider when using OAuth2LoginConfigurer

Expected Behavior

Inside OAuth2LoginConfigurer during init method when postProcess method invoked for OidcAuthorizationCodeAuthenticationProvider do not cast it to OidcAuthorizationCodeAuthenticationProvider but use interface AuthenticationProvider to provide ability to decorate provider inside custom ObjectPostProcessor.

Current Behavior
Inside OAuth2LoginConfigurer during init method when postProcess method invoked for OidcAuthorizationCodeAuthenticationProvider the result object is casted to OidcAuthorizationCodeAuthenticationProvider

Code reference:

OAuth2UserService<OidcUserRequest, OidcUser> oidcUserService = getOidcUserService();
OidcAuthorizationCodeAuthenticationProvider oidcAuthorizationCodeAuthenticationProvider = new OidcAuthorizationCodeAuthenticationProvider(accessTokenResponseClient, oidcUserService);
			.....
oidcAuthorizationCodeAuthenticationProvider = this.postProcess(oidcAuthorizationCodeAuthenticationProvider);
http.authenticationProvider(oidcAuthorizationCodeAuthenticationProvider);

Context

I want to decorate AuthenticationProvider behaviour to invoke some logic before main provider and after.
Alternative is to extend OidcAuthorizationCodeAuthenticationProvider and make a decorator from this class but it looks bad, because i need to inject non null accessTokenResponseClient, oidcUserService parameters (some dummy implementations). Based on code improvement shouldn't be complex.

Start in OAuth2LoginConfigurer's init method at the ObjectPostProcessor call for OidcAuthorizationCodeAuthenticationProvider. Trace how the resulting AuthenticationProvider is registered with http.authenticationProvider, then verify that a custom ObjectPostProcessor can decorate it without requiring concrete-provider parameters; confirm the relevant OAuth2 login tests still pass.