Extending jar task via bundle convention plugin is deprecated

[pun-ky]

1. https://github.com/wttech/gradle-aem-plugin/blob/master/src/main/kotlin/com/cognifide/gradle/aem/bundle/tasks/BundleJar.kt#L404
2. https://docs.gradle.org/current/javadoc/org/gradle/api/Task.html#getConvention

[pun-ky]

impl details: introduce in AemExtension a map <Jar, BundleJar> then add Jar.bundle extension method which will add BundleJar decorator into that map and allow decorating Jar using its own DSL. what will change is the memory allocation place, not directly in Jar task but in AemExtension map.

[pun-ky]

related to #885