Allow set the servlet registration in the TLDScanner instead of the constant "jsp"

In the TLDScanner, it searches for the "jsp" servlet registration.

```java
ServletRegistration reg = ctxt.getServletRegistration("jsp");
```

While it's not wrong, it would be better if it was possible to change the servlet it searches for.

https://github.com/eclipse-ee4j/wasp/blob/master/src/main/java/org/apache/jasper/runtime/TldScanner.java#L164