Support for pkl-maven-plugin to generate pkl classes from pkl files in Maven projects

[salaboy]

Hi Folks, I've prototyped a pkl-maven-plugin to mimic what is being done in Gradle.

I can see people already asking for this here: apple/pkl-spring#2,
and this example show how it should be wired, but it didn't add a new pkl-maven-plugin artifact that simplify the wiring: https://github.com/apple/pkl-spring/pull/49/changes

A plugin like this highly simplify the work that users needs to do when working with maven based projects.

Is it ok if I send a PR with these changes?

I've created the plugin with an example in this repository: https://github.com/salaboy/spring-boot-pkl/