Add missing imports to HaGatewayProviderModule

After merge, code uses ImmutableList, MonitorConfiguration, and List
but imports were missing causing compilation failures.

- Add import: com.google.common.collect.ImmutableList (for getClusterStatsObservers)
- Add import: io.trino.gateway.ha.config.MonitorConfiguration (for getMonitorConfiguration)
- Add import: java.util.List (for method return type)

Author: hpopuri2

gateway-ha/src/main/java/io/trino/gateway/ha/module/HaGatewayProviderModule.java

@@ -13,6 +13,7 @@
  */
 package io.trino.gateway.ha.module;
 
+import com.google.common.collect.ImmutableList;
 import com.google.inject.AbstractModule;
 import com.google.inject.Provides;
 import com.google.inject.Scopes;
@@ -37,6 +38,7 @@
 import io.trino.gateway.ha.config.DataStoreConfiguration;
 import io.trino.gateway.ha.config.GatewayCookieConfigurationPropertiesProvider;
 import io.trino.gateway.ha.config.HaGatewayConfiguration;
+import io.trino.gateway.ha.config.MonitorConfiguration;
 import io.trino.gateway.ha.config.OAuth2GatewayCookieConfigurationPropertiesProvider;
 import io.trino.gateway.ha.config.RoutingRulesConfiguration;
 import io.trino.gateway.ha.config.RulesExternalConfiguration;
@@ -69,6 +71,8 @@
 import org.jdbi.v3.core.Jdbi;
 import org.jdbi.v3.sqlobject.SqlObjectPlugin;
 
+import java.util.List;
+
 import static com.google.inject.multibindings.Multibinder.newSetBinder;
 import static io.airlift.jaxrs.JaxrsBinder.jaxrsBinder;
 import static io.trino.gateway.ha.config.ClusterStatsMonitorType.INFO_API;