Consider an equivalent of upstream 'ExtendsAutoValue' for Immutables

[carterkozak]

See https://errorprone.info/bugpattern/ExtendsAutoValue code: https://github.com/google/error-prone/blob/master/core/src/main/java/com/google/errorprone/bugpatterns/ExtendsAutoValue.java

What happened?

The same justification applies to immutables, but it would be helpful to provide a check at build time. I've fixed bugs caused by this where a threadpool was created in a default method and assumed to be cached, but the human-generated implementation failed to follow the annotation contract.

What did you want to happen?

fail compilation.