Allow configuration of AutoFactory to use custom instantiator instead of new

Gradle recommends using [`ObjectFactory#newInstance`](https://github.com/gradle/gradle/blob/048958212baea839816c621f60db1975578e525a/subprojects/core-api/src/main/java/org/gradle/api/model/ObjectFactory.java#L91) to create new instances of classes as it performs a runtime decoration on the classes. It takes the class to instantiate as the first argument and all of the `@Inject` annotated constructor arguments as the following arguments.

Unfortunately, AutoFactory is opinionated to use `new` (with good reason). I'm wondering if we could accommodate cases such as this one where a custom instantiator could be used instead of using `new`. I would rather not have to maintain a fork of this processor.

Is this something this project would accept as a contribution? If so, which design should we go for? It would be nice to `@Provided` the instantiator class so it can be used automatically.