Previously, @Grab annotations would use Ivy to download the
dependencies with some of Ivy's known limitations being worked around
by GrapeEngineCustomizer.

This commit adds a GrapeEngine implementation that uses Aether,
the dependency resolution 'engine' used by Maven and Grails. To ensure
consistent behaviour with a Maven build, the Aether-powered dependency
resolution uses the dependency management configuration from the
spring-boot-starter-parent pom file.