apply license gradle plugin (#13535)

This PR applies the [Gradle License Plugin](https://github.com/hierynomus/license-gradle-plugin).
The plugin contributes a gradle task named `downloadLicenses` which generates a license report of every dependency.

The plugin is applied to all subprojects and the root project to get an aggregated report.

./gradlew downloadLicenses
open build/reports/license/license-dependency.html

build.gradle

@@ -11,6 +11,7 @@ buildscript {
         classpath "javax.xml.bind:jaxb-api:$jaxbApiVersion"
         classpath "com.sun.xml.bind:jaxb-impl:$jaxbImplVersion"
         classpath "org.codehaus.groovy.modules.http-builder:http-builder:0.7.2"
+        classpath("gradle.plugin.com.hierynomus.gradle.plugins:license-gradle-plugin:0.16.1")
     }
 }
 
@@ -295,6 +296,7 @@ allprojects {
             }
         }
     }
+    apply from: rootProject.layout.projectDirectory.file('gradle/dependency-licenses.gradle')
 }
 
 apply from: "gradle/idea.gradle"

gradle/dependency-licenses.gradle

@@ -0,0 +1,11 @@
+apply plugin: 'com.github.hierynomus.license-report'
+
+List<String> licenseExclusions = rootProject.subprojects.collect {
+    "org.grails:${it.name}:${rootProject.projectVersion}" as String
+}
+
+downloadLicenses {
+    includeProjectDependencies = true
+    dependencyConfiguration = 'runtimeClasspath'
+    excludeDependencies = licenseExclusions
+}