[Tooling] Add google owned cache for dependencies as an option in ci (#4567)

Proof of concept of a package using artifact hub.
Artifact hub is a google owned and managed cache of google/maven dependencies. 
Using this cache should decrease the number of flakes related to downloading dependencies along with future benefits of licence analysis and security alerts. 

Read more at go/artifact-hub#maven
flutter/flutter/issues/120119 

Nest steps:
Enable a new env variable on CI servers. https://flutter-review.googlesource.com/c/recipes/+/48260 cl/551888350
Merge this PR. 
Apply this change to all packages 
Add enforcement as part of the gradle check. 

Developers have a new dependency for buildscript but it is publicly available, so I added a next changelog.

packages/camera/camera_android_camerax/example/android/build.gradle

@@ -16,6 +16,12 @@ buildscript {
 
 allprojects {
     repositories {
+        // See https://github.com/flutter/flutter/wiki/Plugins-and-Packages-repository-structure#gradle-structure for more info.
+        def artifactRepoKey = 'ARTIFACT_HUB_REPOSITORY'
+        if (System.getenv().containsKey(artifactRepoKey)) {
+            println "Using artifact hub"
+            maven { url System.getenv(artifactRepoKey) }
+        }
         google()
         mavenCentral()
     }

packages/camera/camera_android_camerax/example/android/settings.gradle

@@ -9,3 +9,16 @@ localPropertiesFile.withReader("UTF-8") { reader -> properties.load(reader) }
 def flutterSdkPath = properties.getProperty("flutter.sdk")
 assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
 apply from: "$flutterSdkPath/packages/flutter_tools/gradle/app_plugin_loader.gradle"
+
+// See https://github.com/flutter/flutter/wiki/Plugins-and-Packages-repository-structure#gradle-structure for more info.
+buildscript {
+  repositories {
+    maven {
+      url "https://plugins.gradle.org/m2/"
+    }
+  }
+  dependencies {
+    classpath "gradle.plugin.com.google.cloud.artifactregistry:artifactregistry-gradle-plugin:2.2.1"
+  }
+}
+apply plugin: "com.google.cloud.artifactregistry.gradle-plugin"