Improve extensibility in Pre-Authentication scenarios

We are using GeoServer in a Pre-Authentication scenario. GeoServerPreAuthenticationFilter is used as a basis for an own implementation which performs custom logic for retrieval of roles and also uses a custom principal implementation for certain reasons.

However currently we have to patch GeoServerPreAuthenticationFilter to integrate the required logic. Through a very small adjustment projects depending on custom login can maintain their stack more easily.

I will propose a pull request to introduce a small factory method within PreAuthenticatedAuthenticationToken, rather than using the constructor for the token inline.

Thanks for considering this and best regards,

Andreas