war:exploded should have an explicit incremental option

### New feature, improvement proposal

When checking whether to copy files into the exploded war, the plugin is aggressively copying each file even if it is already present with an modification date that indicates that it doesn't need to be fully refreshed. This can be a significant slowdown in larger projects (For context, I'm using this on a large legacy java application where the entire non-exploded war file is about half a GB and contains ~21k files of which 787 are separate JARs, and copying the entire set in can take ~2 minutes (which I am entirely aware is probably limited by other factors outside of this plugin!)).

Forking the plugin locally and making myself a version where I just universally change the copyFile invocations in AbstractWarPackagingTask to all have onlyIfModified = true, then the overall WAR compilation time is cut down to seconds and the final output exploded war is equivalent and consistently gets updated as I make changes.

This seems like a fairly simple feature to add that has marginal benefits to people in a similar situation