feat: add index build state as readiness check indicator (#6700)

#### What type of PR is this?
/kind improvement
/area core
/milestone 2.20.x

#### What this PR does / why we need it:
将索引构建状态添加到就绪检测的指标中

#### Which issue(s) this PR fixes:
Fixes #6632

#### Does this PR introduce a user-facing change?
```release-note
将索引构建状态添加到就绪检测的指标中以优化就绪时访问出现索引不可用的问题
```

application/src/main/java/run/halo/app/extension/ReactiveExtensionClientImpl.java

@@ -19,6 +19,8 @@
 import lombok.NonNull;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
+import org.springframework.boot.availability.AvailabilityChangeEvent;
+import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.context.event.ContextRefreshedEvent;
 import org.springframework.context.event.EventListener;
 import org.springframework.dao.DataIntegrityViolationException;
@@ -30,6 +32,7 @@
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.util.retry.Retry;
+import run.halo.app.extension.availability.IndexBuildState;
 import run.halo.app.extension.exception.ExtensionNotFoundException;
 import run.halo.app.extension.index.DefaultExtensionIterator;
 import run.halo.app.extension.index.ExtensionIterator;
@@ -443,6 +446,7 @@ class IndexBuildsManager {
         private final ExtensionConverter converter;
         private final ReactiveExtensionStoreClient client;
         private final SchemeWatcherManager schemeWatcherManager;
+        private final ApplicationEventPublisher eventPublisher;
 
         @NonNull
         private ExtensionIterator<Extension> createExtensionIterator(Scheme scheme) {
@@ -459,6 +463,8 @@ private ExtensionIterator<Extension> createExtensionIterator(Scheme scheme) {
 
         @EventListener(ContextRefreshedEvent.class)
         public void startBuildingIndex() {
+            AvailabilityChangeEvent.publish(eventPublisher, this, IndexBuildState.BUILDING);
+
             final long startTimeMs = System.currentTimeMillis();
             log.info("Start building index for all extensions, please wait...");
             schemeManager.schemes()
@@ -474,6 +480,8 @@ public void startBuildingIndex() {
                     indexerFactory.removeIndexer(scheme);
                 }
             });
+
+            AvailabilityChangeEvent.publish(eventPublisher, this, IndexBuildState.BUILT);
             log.info("Successfully built index in {}ms, Preparing to lunch application...",
                 System.currentTimeMillis() - startTimeMs);
         }

application/src/main/java/run/halo/app/extension/availability/IndexBuildState.java

@@ -0,0 +1,8 @@
+package run.halo.app.extension.availability;
+
+import org.springframework.boot.availability.AvailabilityState;
+
+public enum IndexBuildState implements AvailabilityState {
+    BUILDING,
+    BUILT;
+}

application/src/main/java/run/halo/app/extension/availability/IndexBuildStateHealthIndicator.java

@@ -0,0 +1,22 @@
+package run.halo.app.extension.availability;
+
+import org.springframework.boot.actuate.availability.AvailabilityStateHealthIndicator;
+import org.springframework.boot.actuate.health.Status;
+import org.springframework.boot.availability.ApplicationAvailability;
+import org.springframework.stereotype.Component;
+
+@Component
+public class IndexBuildStateHealthIndicator extends AvailabilityStateHealthIndicator {
+    /**
+     * Create a {@link IndexBuildStateHealthIndicator} instance by {@link ApplicationAvailability}.
+     * Mapping {@link IndexBuildState} to {@link Status}.
+     *
+     * @see IndexBuildState
+     */
+    public IndexBuildStateHealthIndicator(ApplicationAvailability availability) {
+        super(availability, IndexBuildState.class, (statusMappings) -> {
+            statusMappings.add(IndexBuildState.BUILT, Status.UP);
+            statusMappings.add(IndexBuildState.BUILDING, Status.OUT_OF_SERVICE);
+        });
+    }
+}