First cut at actions for dependency-graph

- Dependency graph init-script references published version of plugin jar.
- `dependency-graph-generate` action will:
     - Provision Gradle if required
     - Execute Gradle with dependency-graph plugin to generate graph JSON
     - Upload dependency-graph JSON file as workflow artifact
- `dependency-graph-submit` action will:
    - Download dependency-graph JSON artifact
    - Submit the graph via the GitHub dependency submission API

src/resources/init-scripts/github-dependency-graph.init.gradle

@@ -0,0 +1,12 @@
+import org.gradle.github.GitHubDependencyGraphPlugin
+initscript {
+  repositories {
+    maven {
+      url = uri("https://plugins.gradle.org/m2/")
+    }
+  }
+  dependencies {
+    classpath("org.gradle:github-dependency-graph-gradle-plugin:+")
+  }
+}
+apply plugin: GitHubDependencyGraphPlugin