These can be used to more reliably discover whether the user
has declared a function with flux types or "bare" POJOs. They
then pave the way to supporting single valued types in a special
way.
Also consolidate and simplify the logic in FunctionInspector
A bean of type FunctionRegistration registers the function with
user-specified name and other properties, rather than relying on the
bean name.
Alternatively, function catalog keys can be specified as a
@Qualifier, which will be used instead of the bean name if
no registration is found.