Expose default error/success handlers in OAuth2AuthorizationEndpointFilter

[OrangeDog]

Expected Behavior
The default behaviour of the AuthenticationSuccessHandler and AuthenticationFailureHandlers should be acessible, so that they can be extended and/or composed without having to copy-paste them from the current source code.

Current Behavior
In OAuth2AuthorizationEndpointFilter, for example, both handlers are private method references, with no getters defined either.

Context
I wanted to change the error page handler when the redirectUri cannot be used. This was easily possible in the previous implementation.

[jgrandja]

Related gh-1557