Add support for configuring Aether via settings.xml

Previously, Aether's configuration was largely hard-coded making it
impossible to configure a mirror, provide credentials for accessing
a repository, etc.

This commit adds support for configuring Aether via Maven's
settings.xml file. The support is optional and must be enabled by
grabbing spring-boot-maven-settings in an init script. The Aether
instance that's used when running the application will then be
configured using settings.xml. The settings file is expected to be
found in ${user.home}/.m2/settings.xml.

The configuration of the following items is currently supported:

 - Offline
 - Proxies
 - Mirrors
 - Server authentication
 - Local repository location

If the support is not enabled, settings.xml does not exist, or
settings.xml does not configure certain things then sensible defaults
are applied.

spring-boot-cli/src/test/java/org/springframework/boot/cli/compiler/grape/AetherGrapeEngineTests.java

@@ -18,11 +18,11 @@ package org.springframework.boot.cli.compiler.grape;
 
 import groovy.lang.GroovyClassLoader;
 
+import java.net.URI;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
 
-import org.eclipse.aether.repository.RemoteRepository;
 import org.junit.Test;
 
 import static org.junit.Assert.assertEquals;
@@ -36,9 +36,9 @@ public class AetherGrapeEngineTests {
 
 	private final GroovyClassLoader groovyClassLoader = new GroovyClassLoader();
 
-	private final AetherGrapeEngine grapeEngine = new AetherGrapeEngine(
-			this.groovyClassLoader, Arrays.asList(new RemoteRepository.Builder("central",
-					"default", "http://repo1.maven.org/maven2/").build()));
+	private final AetherGrapeEngine grapeEngine = AetherGrapeEngineFactory.create(
+			this.groovyClassLoader, Arrays.asList(new RepositoryConfiguration("central",
+					URI.create("http://repo1.maven.org/maven2"), false)));
 
 	@Test
 	public void dependencyResolution() {