Add test for component templates updated after cluster restart

This commit adds an integration test that component templates used to form a composite template can
still be updated after a cluster restart.

In elastic#58643 an issue arose where mappings were causing problems because of the way we unwrap `_doc` in
template mappings. This was also related to the mappings being merged manually rather than using the
`MapperService` to do the merging. elastic#58643 was fixed in 7.9 and master with the elastic#58521 change, since
mappings now are read and digested by the actual mapper service.

This test passes for 7.x and master, and I intend to open a separate PR including this test for
7.8.1 along with a bug fix for elastic#58643. This test is to ensure we don't have any regression in the
future.

server/src/internalClusterTest/java/org/elasticsearch/indices/template/ComposableTemplateIT.java

@@ -0,0 +1,86 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.indices.template;
+
+import org.elasticsearch.action.admin.indices.template.put.PutComponentTemplateAction;
+import org.elasticsearch.action.admin.indices.template.put.PutComposableIndexTemplateAction;
+import org.elasticsearch.cluster.metadata.ComponentTemplate;
+import org.elasticsearch.cluster.metadata.ComposableIndexTemplate;
+import org.elasticsearch.cluster.metadata.Template;
+import org.elasticsearch.common.compress.CompressedXContent;
+import org.elasticsearch.test.ESIntegTestCase;
+
+import java.util.Collections;
+
+public class ComposableTemplateIT extends ESIntegTestCase {
+
+    // See: https://github.com/elastic/elasticsearch/issues/58643
+    public void testComponentTemplatesCanBeUpdatedAfterRestart() throws Exception {
+        ComponentTemplate ct = new ComponentTemplate(new Template(null, new CompressedXContent("{\n" +
+            "      \"dynamic\": false,\n" +
+            "      \"properties\": {\n" +
+            "        \"foo\": {\n" +
+            "          \"type\": \"text\"\n" +
+            "        }\n" +
+            "      }\n" +
+            "    }"), null), 3L, Collections.singletonMap("eggplant", "potato"));
+        client().execute(PutComponentTemplateAction.INSTANCE, new PutComponentTemplateAction.Request("my-ct").componentTemplate(ct)).get();
+
+        ComposableIndexTemplate cit = new ComposableIndexTemplate(Collections.singletonList("coleslaw"),
+            new Template(null, new CompressedXContent("{\n" +
+                "      \"dynamic\": false,\n" +
+                "      \"properties\": {\n" +
+                "        \"foo\": {\n" +
+                "          \"type\": \"keyword\"\n" +
+                "        }\n" +
+                "      }\n" +
+                "    }"), null), Collections.singletonList("my-ct"),
+            4L, 5L, Collections.singletonMap("egg", "bread"));
+        client().execute(PutComposableIndexTemplateAction.INSTANCE,
+            new PutComposableIndexTemplateAction.Request("my-it").indexTemplate(cit)).get();
+
+        internalCluster().fullRestart();
+        ensureGreen();
+
+        ComponentTemplate ct2 = new ComponentTemplate(new Template(null, new CompressedXContent("{\n" +
+            "      \"dynamic\": true,\n" +
+            "      \"properties\": {\n" +
+            "        \"foo\": {\n" +
+            "          \"type\": \"keyword\"\n" +
+            "        }\n" +
+            "      }\n" +
+            "    }"), null), 3L, Collections.singletonMap("eggplant", "potato"));
+        client().execute(PutComponentTemplateAction.INSTANCE,
+            new PutComponentTemplateAction.Request("my-ct").componentTemplate(ct2)).get();
+
+        ComposableIndexTemplate cit2 = new ComposableIndexTemplate(Collections.singletonList("coleslaw"),
+            new Template(null, new CompressedXContent("{\n" +
+                "      \"dynamic\": true,\n" +
+                "      \"properties\": {\n" +
+                "        \"foo\": {\n" +
+                "          \"type\": \"integer\"\n" +
+                "        }\n" +
+                "      }\n" +
+                "    }"), null), Collections.singletonList("my-ct"),
+            4L, 5L, Collections.singletonMap("egg", "bread"));
+        client().execute(PutComposableIndexTemplateAction.INSTANCE,
+            new PutComposableIndexTemplateAction.Request("my-it").indexTemplate(cit2)).get();
+    }
+}