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.

spring-boot-parent/pom.xml

@@ -38,6 +38,11 @@
 				<artifactId>ivy</artifactId>
 				<version>2.3.0</version>
 			</dependency>
+			<dependency>
+				<groupId>org.apache.maven</groupId>
+				<artifactId>maven-aether-provider</artifactId>
+				<version>3.1.0</version>
+			</dependency>
 			<dependency>
 				<groupId>org.apache.maven</groupId>
 				<artifactId>maven-archiver</artifactId>
@@ -103,6 +108,21 @@
 				<artifactId>plexus-utils</artifactId>
 				<version>3.0.10</version>
 			</dependency>
+			<dependency>
+      			<groupId>org.eclipse.aether</groupId>
+      			<artifactId>aether-api</artifactId>
+      			<version>${aether.version}</version>
+   			</dependency>
+   			<dependency>
+      			<groupId>org.eclipse.aether</groupId>
+      			<artifactId>aether-impl</artifactId>
+      			<version>${aether.version}</version>
+   			</dependency>
+    		<dependency>
+      			<groupId>org.eclipse.aether</groupId>
+      			<artifactId>aether-util</artifactId>
+      			<version>${aether.version}</version>
+    		</dependency>
 			<dependency>
 				<groupId>org.gradle</groupId>
 				<artifactId>gradle-core</artifactId>