Support client controlled contexts

In a client controlled context, the client controls via some identifier (probably an identifier, but we should allow the context to inspect the injection point) which contextual instances it sees for a particular bean type. For example given:

@MyScope
class Foo {

   String name;

}

and a context which uses annotations to differentiate between contextual instance, these two injection points would see different instances:

@Inject @Bar Foo barFoo;

@Inject @Baz Foo bazFoo;