External Configuration File for Spotless Maven Plugin

I am currently using the Spotless Maven Plugin in my Java projects to enforce code formatting and have found its features extremely useful in maintaining a clean, consistent codebase.

However, I have found a limitation in the current version of the plugin that I believe could be greatly beneficial if addressed. Currently, there's no direct way to read Spotless configuration from an external XML file in the Spotless Maven Plugin. Configurations for the plugin are currently provided directly within the tag of the pom.xml file of the project.

This can pose a challenge in maintaining consistency of configurations across multiple projects. Although sharing a common parent pom file is an option, it is not always viable, particularly when projects belong to different hierarchies or when complete control over formatting rules is required.

Spotless Maven Plugin could read configuration from a file (local or URL) and treat all other explicit configuration parameters as overrides.

For example we do not have something as shown below:
```

com.diffplug.spotless
spotless-maven-plugin
${spotless.version}

checkstyle.xml


```
We could have this similar to how [maven-checkstyle-plugin](https://maven.apache.org/plugins/maven-checkstyle-plugin/examples/custom-checker-config.html) supports this.

Consequently, I would like to propose the addition of a feature for reading the Spotless configuration from an external XML (or properties) file. I believe that this feature will not only enhance the flexibility and control over code formatting rules but also improve the experience of using Spotless in cases where we want to share specific configurations across multiple, separate projects.

I hope you'll consider my proposal for this feature. I am convinced that its inclusion can significantly enhance the plugin's capabilities.

CC: @nedtwigg @lutovich

Inizia esaminando la configurazione di Spotless Maven Plugin in pom.xml e l'esempio di configurazione esterna di Maven Checkstyle collegato nell'issue. Determina come un configLocation locale o basato su URL potrebbe fornire valori predefiniti, mentre una configurazione esplicita lo sovrascrive; il lavoro è completato quando il plugin accetta il file esterno e mantiene il comportamento esistente della configurazione inline.

Scritto dal modello di indicizzazione a partire dal testo della issue.