Consolidate logic for merging authorities

[jzheaux]

Related to #18021, it would be nice if Authentication.Builder could merge authorities on its own. One way to do this is to add an authentication method to the builder, which would by default merge authorities from the given authentication into the current builder instance.

This will consolidate the logic currently residing in a half-dozen filters that are currently performing this authority-merging logic.

[jzheaux]

Let's not add this so close to RC1 as it's not needed. We can add after 7 releases.