Add a convenience method to combine a ServiceDependency with another.
This is analogous to CompletionStage.thenCombine(...).
e.g.
ServiceDependency<Foo> foo = ...;
ServiceDependency<Bar> bar = ...;
BiFunction<Foo, Bar, Qux> mapper = ...;
ServiceDependency<Qux> qux = foo.combine(bar, mapper);

The resulting service dependency will register a dependency on both foo and bar, while returning qux.