Avoid reflection in `PathFunctionFactory`

[odrotbohm]

As reported in spring-projects/spring-modulith#1250, PathFunctionFactory invokes the default constructors of the function types registered by reflection. This causes issues on GraalVM unless you explicitly register those constructors for reflection.

Could the usage of reflection be avoided in general by tweaking the Map to rather use Supplier<PathFunction> as values and register the instance creation as () -> new KeySetFunction() etc.?