Cannot load XSLT custom function from a class in the current application

[ppalaga]

A followup of #3526 (comment)

There is a known limitation in #3526 that custom function classes have to be defined in a separate jar for the transformer to be able to load them. I think this is caused by the fact that TransformerFactory class is resolved using a wrong class loader around here in Camel https://github.com/apache/camel/blob/camel-3.15.0/components/camel-xslt/src/main/java/org/apache/camel/component/xslt/XsltEndpoint.java#L372
Perhaps changing that (or better making configurable) to current thread class loader could solve the problem.

Here are some changes needed to test the potential fix: https://github.com/ppalaga/camel-quarkus/commits/i3534

[JiriOndrusek]

@ppalaga At the first glance, problem seems to be a little bit complicated.
I debugged the test for the correct behavior and problem is caused by the fact, that quarkus 's classloader bans the class from the test module in this line.
Workaround is to use quarkus.test.flat-class-path=true.

I'm investigating the reason (which at the first glance seems like a bug on quarkus or an intentional feature, which is not reflected by the test)