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

Start by inspecting the referenced pkl-spring-boot-starter/pom.xml and the existing pkl-spring artifact. Add the starter as an aggregator artifact following the Spring Boot naming convention, bringing together the required pkl-spring dependencies. Done means a Spring user can discover and include pkl-spring-boot-starter as the single starter dependency.

Written by the indexing model from the issue text.