Implement framework for migrating system indices (#78951)

This PR adds a framework for migrating system indices as necessary prior
to Elasticsearch upgrades. This framework uses REST APIs added in
another commit:
- GET _migration/system_features

This API, which gets the status of "features" (plugins which own system
indices) with regards to whether they need to be upgraded or not. As of
this PR, this API also reports errors encountered while migrating system
indices alongside the index that was being processed when this occurred.

As an example of this error reporting:

```json
{
    "feature_name": "logstash_management",
    "minimum_index_version": "8.0.0",
    "upgrade_status": "ERROR",
    "indices": [
        {
            "index": ".logstash",
            "version": "8.0.0",
            "failure_cause": {
                "error": {
                    "root_cause": [
                        {
                            "type": "runtime_exception",
                            "reason": "whoopsie",
                            "stack_trace": "<omitted for brevity>"
                        }
                    ],
                    "type": "runtime_exception",
                    "reason": "whoopsie",
                    "stack_trace": "<omitted for brevity>"
                }
            }
        }
    ]
}
```

- POST _migration/system_features

This API starts the migration process. The API for this has no changes,
but when called, any system indices which need to be migrated will be
migrated, with status information stored in the cluster state for later
use by the GET _migration/system_features API.

client/rest-high-level/src/test/java/org/elasticsearch/client/MigrationIT.java

@@ -28,8 +28,8 @@
 import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThanOrEqualTo;
+import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.is;
-import static org.hamcrest.Matchers.nullValue;
 
 public class MigrationIT extends ESRestHighLevelClientTestCase {
 
@@ -73,11 +73,8 @@ public void testGetFeatureUpgradeStatus() throws IOException {
     public void testPostFeatureUpgradeStatus() throws IOException {
         PostFeatureUpgradeRequest request = new PostFeatureUpgradeRequest();
         PostFeatureUpgradeResponse response = highLevelClient().migration().postFeatureUpgrade(request, RequestOptions.DEFAULT);
-        assertThat(response.isAccepted(), equalTo(true));
-        assertThat(response.getFeatures().size(), equalTo(1));
-        PostFeatureUpgradeResponse.Feature feature = response.getFeatures().get(0);
-        assertThat(feature.getFeatureName(), equalTo("security"));
-        assertThat(response.getReason(), nullValue());
-        assertThat(response.getElasticsearchException(), nullValue());
+        assertThat(response.isAccepted(), equalTo(false));
+        assertThat(response.getFeatures(), hasSize(0));
+        assertThat(response.getReason(), equalTo("No system indices require migration"));
     }
 }

client/rest-high-level/src/test/java/org/elasticsearch/client/migration/GetFeatureUpgradeStatusResponseTests.java

@@ -40,10 +40,14 @@ protected org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStat
                     randomAlphaOfLengthBetween(3, 20),
                     randomFrom(Version.CURRENT, Version.CURRENT.minimumCompatibilityVersion()),
                     randomFrom(org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStatusResponse.UpgradeStatus.values()),
-                    randomList(4,
-                        () -> new org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStatusResponse.IndexVersion(
+                    randomList(
+                        4,
+                        () -> new org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStatusResponse.IndexInfo(
                             randomAlphaOfLengthBetween(3, 20),
-                            randomFrom(Version.CURRENT, Version.CURRENT.minimumCompatibilityVersion())))
+                            randomFrom(Version.CURRENT, Version.CURRENT.minimumCompatibilityVersion()),
+                            null
+                        )
+                    )
                 )),
             randomFrom(org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStatusResponse.UpgradeStatus.values())
         );
@@ -78,12 +82,12 @@ protected void assertInstances(
             assertThat(clientStatus.getIndexVersions(), hasSize(serverTestStatus.getIndexVersions().size()));
 
             for (int j = 0; i < clientStatus.getIndexVersions().size(); i++) {
-                org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStatusResponse.IndexVersion serverIndexVersion
+                org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStatusResponse.IndexInfo serverIndexInfo
                     = serverTestStatus.getIndexVersions().get(j);
                 GetFeatureUpgradeStatusResponse.IndexVersion clientIndexVersion = clientStatus.getIndexVersions().get(j);
 
-                assertThat(clientIndexVersion.getIndexName(), equalTo(serverIndexVersion.getIndexName()));
-                assertThat(clientIndexVersion.getVersion(), equalTo(serverIndexVersion.getVersion().toString()));
+                assertThat(clientIndexVersion.getIndexName(), equalTo(serverIndexInfo.getIndexName()));
+                assertThat(clientIndexVersion.getVersion(), equalTo(serverIndexInfo.getVersion().toString()));
             }
         }
     }

docs/reference/migration/apis/feature_upgrade.asciidoc

@@ -24,7 +24,7 @@ and to trigger an automated system upgrade that might potentially involve downti
 ==== {api-prereq-title}
 
 * If the {es} {security-features} are enabled, you must have the `manage`
-<<privileges-list-cluster,cluster privilege>> to use this API. (TODO: true?)
+<<privileges-list-cluster,cluster privilege>> to use this API.
 
 [[feature-upgrade-api-example]]
 ==== {api-examples-title}
@@ -144,6 +144,7 @@ Example response:
   ]
 }
 --------------------------------------------------
+// TESTRESPONSE[skip: can't actually upgrade system indices in these tests]
 
 This tells us that the security index is being upgraded. To check the
 overall status of the upgrade, call the endpoint with GET.