Add Support for customizing restOperations on OidcBackChannelLogoutHandler

Expected Behavior

We would like to set restOperations (RestTemplate) on OidcBackChannelLogoutHandler.

Current Behavior

Currently restOperations cannot be directly modified on OidcBackChannelLogoutHandler requiring us to copy the entire classes' source only to change restOperations.

Context

We need to customize this because we use locally signed certificates internally and do SSL offloading later, so when the Spring Boot app tries to call itself on localhost, it fails because it does not trust this locally signed certificate.

Of course there are workarounds for this problem such as configuring a JDK wide trust store but it would be really helpful to have native support for this especially considerung that the restOperations can be configured on other OIDC components as well.

Start with config/src/main/java/org/springframework/security/config/annotation/web/configurers/oauth2/client/OidcBackChannelLogoutHandler.java and inspect how its restOperations is initialized and used. Compare nearby OIDC components that already allow RestTemplate customization. Done means callers can provide custom restOperations for back-channel logout without copying the class, including support for locally signed certificates.