Add authenticationSuccessHandler for OAuth2 auth code callback

[wonderfulrosemari]

Closes gh-11069

This PR adds support for configuring a custom AuthenticationSuccessHandler
for the OAuth2 authorization code callback filter.

Changes include:

• adding setAuthenticationSuccessHandler(...) to OAuth2AuthorizationCodeGrantFilter
• invoking the custom success handler after saving the authorized client
• preserving existing redirect behavior when no custom success handler is configured
• adding tests for null validation and custom success handler invocation
• exposing the customization through OAuth2ClientConfigurer.AuthorizationCodeGrantConfigurer
• adding config tests to verify DSL wiring