Cleanup ServletHandler, specifically with respect to making filter chains more extensible

[joakime]

Jetty version
9.4.30

Description
The ServletHandler has a method for overriding a CachedChain via the newCachedChain() method.

https://github.com/eclipse/jetty.project/blob/ae43b70a9f8907b8a2444f051bebab60f4a9162c/jetty-servlet/src/main/java/org/eclipse/jetty/servlet/ServletHandler.java#L826-L836

But this cannot be used, as the CachedChain inner class is protected.

This method is only called from ServletHandler.getFilterChain().

https://github.com/eclipse/jetty.project/blob/ae43b70a9f8907b8a2444f051bebab60f4a9162c/jetty-servlet/src/main/java/org/eclipse/jetty/servlet/ServletHandler.java#L636-L637

There is also a missing related method for non-cached that should be created in ServletHandler.

public Chain newChain(Request baseRequest, List<FilterHolder> filters, ServletHolder servletHolder)

Which is used in getFilterChain()

https://github.com/eclipse/jetty.project/blob/ae43b70a9f8907b8a2444f051bebab60f4a9162c/jetty-servlet/src/main/java/org/eclipse/jetty/servlet/ServletHandler.java#L660-L661

[gregw]

@joakime does the newCachedChain method need to return a CachedChain? Can it just return a FilterChain ?

[gregw]

If we are going to expose more of this API.... we need to clean it up... in fact this whole class needs a bit of a refresh.... changing title!