Provide an Aether-based Grape Engine

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.
Isolate Aether in a separate class loader

Prior to this commit, the Aether-based GrapeEngine was loaded in the
same class loader as the rest of Boot. This led to Aether's and its
dependencies' types polluting the application's class path. Most
notably, this caused problems with logging as the logging framework
could be permaturely initialized.

This commit isolates AetherGrapeEngine, Aether and its dependencies
into a separate class loader. This is done by customizing the
packaging of the CLI's jar file with the internal directory housing
all of the types that will be loaded by the separate class loader.
Merge branch 'aether-grab'