Add SPDX license header checker and missing headers

.github/workflows/license-header-checker.yml

@@ -0,0 +1,12 @@
+---
+name: License Header Checker
+
+on: [push, pull_request]
+
+jobs:
+  license-header-checker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+      - name: Add License Header
+        run: npx @kt3k/license-checker@3.2.2

.licenserc.json

@@ -0,0 +1,35 @@
+{
+  "**/*.*": [
+    " Copyright OpenSearch Contributors",
+    " SPDX-License-Identifier: Apache-2.0"
+  ],
+  "ignore": [
+    ".md",
+    ".json",
+    ".yml",
+    ".yaml",
+    ".txt",
+    ".xml",
+    ".config",
+    ".png",
+    ".jar",
+    ".gz",
+    ".lock",
+    ".toml",
+    ".ini",
+    ".bat",
+    ".policy",
+    ".properties",
+    ".git",
+    ".gitignore",
+    ".whitesource",
+    ".gradle",
+    "gradle/wrapper",
+    "settings.gradle",
+    "**/build/",
+    "licenses/",
+    "src/test/resources/",
+    "LICENSE",
+    "NOTICE"
+  ]
+}

build.gradle

@@ -1,3 +1,8 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
 import java.nio.file.Files
 import org.opensearch.gradle.testclusters.OpenSearchCluster
 import org.opensearch.gradle.testclusters.StandaloneRestIntegTestTask

formatter/formatting.gradle

@@ -1,3 +1,8 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
 allprojects {
     spotless {
         java {

src/main/resources/META-INF/services/org.opensearch.security.spi.resources.ResourceSharingExtension

@@ -1 +1,4 @@
+# Copyright OpenSearch Contributors
+# SPDX-License-Identifier: Apache-2.0
+
 org.opensearch.flowframework.FlowFrameworkResourceSharingExtension