Add support for tagging an annotation processor

[snicoll]

As of Gradle 4.6, a build with an annotation processor produces the following warning:

Putting annotation processors on the compile classpath has been deprecated and is scheduled to be removed in Gradle 5.0. Please add them to the processor path instead.

We should move annotation processors to the right place, see the documentation.

See spring-projects/spring-boot#12824

Doing so will allow you to implement #438 as well.

[snicoll]

@mbhave good catch not updating the service to use the new scope as it may break users. I've created #705 to see if we should upgrade now.