Allow skipping sortPom

While other formatters allow to set some sort of regular expression to signal which files should be processed, sortPom formatting does not have this option (while sortPom has), [`pom.xml`](https://github.com/diffplug/spotless/blob/main/plugin-maven/src/main/java/com/diffplug/spotless/maven/pom/Pom.java#L35) is hardcoded. This has two drawbacks:
* secondary poms can't be processed by sortPom; i.e., we have some poms that invoke a "secondary" pom in the same folder, which for obvious reasons can't be named `pom.xml`
* it doesn't allow to bypass sortPom formatting. Currently we define some properties at the corporate pom level that allow to disable certain spotless formatters, via the regexs above, so it's easy to skip formatting, or apply it only to a subset of elements

would be nice to have this option for sortPom too. Thanks!