Consider Adding `Authentication.Builder.authentication(Authentication)`

[rwinch]

CAUTION This is a ticket that needs to be considered by the Spring Security team and thus is not currently a candidate for a pull request.

Currently, there is a lot of duplicated logic for merging two Authentication instances that cannot be replaced (see 64c9e3e for the locations).

We should consider adding Authentication.Builder.authentication(Authentication) which can encapsulate the logic and allow custom Authentication.toBuilder() implementations to replace the way in which the Authentication instances are merged.