[FEATURE] Support custom builder methods referring to fields with defaults

[scf37]

Describe the feature
add protected getters to builders so custom builder methods can use them.

Current situation:

@Builder
public final class Response {
    private final int statusCode;
    @Builder.Default private final Headers headers = Headers.newInstance();
    
    public static class ResponseBuilder {
        public ResponseBuilder header(String name, String value) {
            if (!headers$set) headers$value = $default$headers();
            headers$value.put(name, value);
            return this;
        }
    }
}

After the change:

@Builder
public final class Response {
    private final int statusCode;
    @Builder.Default private final Headers headers = Headers.newInstance();
    
    public static class ResponseBuilder {
        public ResponseBuilder header(String name, String value) {
            headers().put(name, value);
            return this;
        }
    }
}

Implementation:

    public static class ResponseBuilder {
        public ResponseBuilder header(String name, String value) {
            headers().put(name, value);
            return this;
        }
        protected Headers headers() { // protected since default Builder is not final - can be extended?
            if (!headers$set) headers$value = $default$headers();
            return headers$value;
        }
    }

Describe the target audience
Anyone needing custom methods in their builders