Add ability to put <scope>provided</scope> artifacts on system classpath for jetty-jspc-maven-plugin

The jetty-maven-plugin has a <useProvidedScope> configuration option which will put all dependencies with <scope>provided</scope> onto the execution classpath.

Although this can be a dangerous option to use, as you can wind up with duplicate classes on the classpath (2 different artifacts containing the same classes, one on the plugin classpath and one on the project classpath for example), it may be a useful option to have also in the jspc plugin.