Introduce a Spring Fu Gradle plugin

[sdeleuze]

This plugin could provide the following functionalities with first-class Gradle Kotlin DSL support for configuration:

• Preconfigure Junit 5 by applying val test by tasks.getting(Test::class) { useJUnitPlatform() } automatically
• Apply dependency management
• Provide shadow plugin like functionalities
• Integrate JAR minimization mechanism + removal of annotation/XML stuff in Spring JARs as done is Spring classpath filtering via Gradle plugin #34
• Preconfigure Kotlin null-safety suppport, bytecode level, etc.

[sdeleuze]

Closed in favor of #68.