Configurations.add(Configuration) results in UnsupportedOperationException

[joakime]

Jetty version(s)
10.0.x+

Java version/vendor (use: java -version)
All

OS type/version
All

Description
While troubleshooting the issue reported at jetty-project/embedded-servlet-server#51

I discovered that if the following line of code is used ...

Configurations.setServerDefault(server).add(new AnnotationConfiguration());

It results in an exception ...

java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractList.add(AbstractList.java:153)
	at java.base/java.util.AbstractList.add(AbstractList.java:111)
	at org.eclipse.jetty.demos.EmbedMe.createServer(EmbedMe.java:61)

This is because the Configurations class only has an overridden add() method that supports Configuration arrays, not single Configuration instances.

https://github.com/eclipse/jetty.project/blob/716c7afd8e40d8f645b32aff06945a249d6eb511/jetty-webapp/src/main/java/org/eclipse/jetty/webapp/Configurations.java#L269

We should also override the AbstractList.add(Configuration) to allow single Configuration instances to be added easier.

[janbartel]

I'm not totally convinced of the necessity for this, as we don't override most of the AbstractList methods. However FWIW here's a PR: #7631