JwtAuthenticationConverter needs a way to merge authorities from UserDetailsService into JwtAuthenticationToken

[edeandrea]

Currently in JwtAuthenticationConverter the authorities passed into new JwtAuthenticationToken come only from the scopes on the token. If the user has set a UserDetailsService or ReactiveUserDetailsService then the authorities returned from userDetailsService.loadByUsername(subject).getAuthorities() / reactiveUserDetailsService.findByUsername(subject).getAuthorities() should be merged into the authorities in the JwtAuthenticationToken.

[edeandrea]

I think what was done in #6273 will solve this as well. It will allow someone to be able to assemble their own JwtGrantedAuthoritiesConverter in whichever way they want.

I'm fine to close this out unless there is more to add. We can re-open at that point.