Allow mappers to expose extended *FactoryContext interfaces to their users

If we were to introduce a <S> generic parameter in the factory contexts in the various DSLs, we might be able to allow this kind of thing:

I suspect the "fluent" syntax was the main problem preventing us from introducing that kind of thing. Now that it's gone, maybe we can improve it.

Some items that could be pulled up to mapper-specific interfaces (just expose the necessary hooks in SPIs)

• .reference() projection

• .object() projection

• .type(Class<?>) predicate (see HSEARCH-3434)

Ideally the <S> type should be propagated to composite contexts (boolean predicates, composite projections, basically all sorts, ...), so that uses of the lambda syntax in these contexts (f.bool().must( f -> ... ) correctly provide an "extended" type to the user.