Add `pkl-spring-boot-starter` for discoverability

Spring Boot starters provide a quick way to add all the dependencies needed by a project with a single (aggregator) artifact. Spring Boot starters use to bring together different autoconfigure packages, in this case pkl-spring brings these configurations. But as a spring user I expect to find a pkl-spring-boot-starter that I can include in my project, as this is the convention adopted by the Spring Boot community.

A simple new artifact like this will do the trick: https://github.com/salaboy/spring-boot-pkl/blob/main/pkl-spring-boot-starter/pom.xml

Notice that I've flagged the Spring Boot version that is already End of Life from a community support perspective in a different issue: https://github.com/apple/pkl-spring/issues/79