Publish Gradle marker artifact for the Spring Boot plugin

[snicoll]

Our plugin coordinates do not match what Gradle expects from the plugin id so some additional code is required to resolve the plugin with a snapshot or milestone version.

Publishing a marker artifact will avoid that and make the setup simpler with Gradle.

[wilkinsona]

This branch contains the changes to publish a marker artifact. It allows the following configuration to be removed from pluginManagement in settings.gradle:

resolutionStrategy {
    eachPlugin {
        if (requested.id.id == 'org.springframework.boot') {
            useModule("org.springframework.boot:spring-boot-gradle-plugin:${requested.version}")
        }
    }
}

The marker artifact could also be of benefit when a release version of the plugin is being used. If the Gradle Plugin Portal is down, the marker artifact will allow it to be resolved from Maven Central if we added mavenCentral() as a repository.