Add Gradle init script to configure artifactoy (#40412)

The script will be use in CI to configure our own Artifactory instance
so CI does not depend on external network resources nor do we generate
excessive load on these resources.
In case the repo is not accessible Gradle should fall back to using the
public ones.

.ci/init.gradle

@@ -0,0 +1,43 @@
+if (System.env.ELASTIC_ARTIFACTORY_USERNAME == null || System.env.ELASTIC_ARTIFACTORY_TOKEN == null) {
+    throw new GradleException("Using init script without configuration")
+} else {
+    logger.info("Using elastic artifactory repos")
+    settingsEvaluated { settings ->
+        settings.pluginManagement {
+            repositories {
+                maven {
+                    url "https://artifactory.elstc.co/artifactory/gradle-plugins"
+                    credentials {
+                        username System.env.ELASTIC_ARTIFACTORY_USERNAME
+                        password System.env.ELASTIC_ARTIFACTORY_TOKEN
+                    }
+                }
+                gradlePluginPortal()
+            }
+        }
+    }
+    projectsLoaded {
+        allprojects {
+            buildscript {
+                repositories {
+                    maven {
+                        url "https://artifactory.elstc.co/artifactory/gradle-release/"
+                        credentials {
+                            username System.env.ELASTIC_ARTIFACTORY_USERNAME
+                            password System.env.ELASTIC_ARTIFACTORY_TOKEN
+                        }
+                    }
+                }
+            }
+            repositories {
+                maven {
+                    url "https://artifactory.elstc.co/artifactory/gradle-release/"
+                    credentials {
+                        username System.env.ELASTIC_ARTIFACTORY_USERNAME
+                        password System.env.ELASTIC_ARTIFACTORY_TOKEN
+                    }
+                }
+            }
+        }
+    }
+}