[improve](alter-job) Add a config for forbiding doing alter job (#23294)

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -2111,4 +2111,7 @@ public class Config extends ConfigBase {
             "是否用 mysql 的 bigint 类型来返回 Doris 的 largeint 类型",
             "Whether to use mysql's bigint type to return Doris's largeint type"})
     public static boolean use_mysql_bigint_for_largeint = false;
+
+    @ConfField
+    public static boolean forbid_running_alter_job = false;
 }

fe/fe-core/src/main/java/org/apache/doris/alter/SchemaChangeHandler.java

@@ -1602,6 +1602,9 @@ public class SchemaChangeHandler extends AlterHandler {
     }
 
     private void runAlterJobV2() {
+        if (Config.forbid_running_alter_job) {
+            return;
+        }
         runnableSchemaChangeJobV2.values().forEach(alterJobsV2 -> {
             if (!alterJobsV2.isDone() && !activeSchemaChangeJobsV2.containsKey(alterJobsV2.getJobId())
                     && activeSchemaChangeJobsV2.size() < MAX_ACTIVE_SCHEMA_CHANGE_JOB_V2_SIZE) {