X509PrincipalExtractor is missing FunctionalInterface annotation

[blake-bauman]

The interface X509PrincipalExtractor is actually a Functional Interface, but it is not annotated with @FunctionalInterface.

This isn't technically a bug, but it was easily missed that this could be expressed as a lambda since the IDE didn't report it as a Functional Interface and we thought we needed an implementation class.

[franticticktick]

Hi @blake-bauman, this ticket will be resolved via #16952.
There is no need to create a separate ticket for such a small polish. We don't need to track such changes. Feel free to contribute.

[jzheaux]

Thanks, @blake-bauman! I'll close this in favor of @franticticktick's PR.