Renaming nested builders

Hi! I'm looking for a way to declare / use nested property builders with names other than default, but can't make it work. I've read https://github.com/google/auto/blob/main/value/userguide/builders-howto.md#accumulate, yet found nothing related.

```java
@AutoValue
public abstract class Animal {
public abstract String name();
public abstract int numberOfLegs();
public abstract ImmutableSet countries();

public static Builder builder() {
return new AutoValue_Animal.Builder();
}

@AutoValue.Builder
public abstract static class Builder {
public abstract Builder setName(String value);
public abstract Builder setNumberOfLegs(int value);
public abstract ImmutableSet.Builder countriesAccumulator(); // <- this doesn't work
public abstract Animal build();
}
}
```

Possibility to use anything other than `fooBuilder()` would be great. This probably means that logic would have to be based entirely on method's return type. This seems to be the way it works in the case of `public static Builder builder()` which can be renamed freely (https://github.com/google/auto/blob/main/value/userguide/builders-howto.md#-use-different-names-besides-builderbuilderbuild).

Ideally, I would like to be able to declare `public abstract ImmutableSet.Builder countries()` with no suffix at all. Currently it leads to name collision with countries getter (as `[AutoValueBuilderReturnType]` nicely explains), but I hope that it is still possible to extend the implementation in a backward compatible way. Distinction between getters and builders seems to be rather unambiguous.