[FEATURE] Refactor plugins.query.executionengine.spark.config

[noCharger]

Is your feature request related to a problem?

If user config the cluster setting incorrectly, the Async Query APIs are disabled.

PUT _cluster/settings
{
  "persistent": {
    "plugins.query.executionengine.spark.config": "{}"
  }
}

[2024-02-06T21:06:16,954][ERROR][o.o.s.s.r.RestAsyncQueryManagementAction] [node-1] Error happened during request handling
java.lang.IllegalArgumentException: Async Query APIs are disabled. Please configure plugins.query.executionengine.spark.config in cluster settings to enable them.
        at org.opensearch.sql.spark.client.EMRServerlessClientFactoryImpl.validateSparkExecutionEngineConfig(EMRServerlessClientFactoryImpl.java:52) ~[spark-2.12.0.0-SNAPSHOT.jar:?]
        at org.opensearch.sql.spark.client.EMRServerlessClientFactoryImpl.getClient(EMRServerlessClientFactoryImpl.java:36) ~[spark-2.12.0.0-SNAPSHOT.jar:?]
        at org.opensearch.sql.spark.dispatcher.SparkQueryDispatcher.dispatch(SparkQueryDispatcher.java:61) ~[spark-2.12.0.0-SNAPSHOT.jar:?]
        at org.opensearch.sql.spark.asyncquery.AsyncQueryExecutorServiceImpl.createAsyncQuery(AsyncQueryExecutorServiceImpl.java:43) ~[spark-2.12.0.0-SNAPSHOT.jar:?]
        at org.opensearch.sql.spark.transport.TransportCreateAsyncQueryRequestAction.doExecute(TransportCreateAsyncQueryRequestAction.java:51) [spark-2.12.0.0-SNAPSHOT.jar:?]
        at org.opensearch.sql.spark.transport.TransportCreateAsyncQueryRequestAction.doExecute(TransportCreateAsyncQueryRequestAction.java:25) [spark-2.12.0.0-SNAPSHOT.jar:?]
        at org.opensearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:218) [opensearch-2.12.0.jar:2.12.0]
        at org.opensearch.indexmanagement.rollup.actionfilter.FieldCapsFilter.apply(FieldCapsFilter.kt:118) [opensearch-index-management-2.12.0.0.jar:2.12.0.0]
        at org.opensearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:216) [opensearch-2.12.0.jar:2.12.0]
        at org.opensearch.indexmanagement.controlcenter.notification.filter.IndexOperationActionFilter.apply(IndexOperationActionFilter.kt:39) [opensearch-index-management-2.12.0.0.jar:2.12.0.0]
        at org.opensearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:216) [opensearch-2.12.0.jar:2.12.0]
        at org.opensearch.security.filter.SecurityFilter.apply0(SecurityFilter.java:395) [opensearch-security-2.12.0.0.jar:2.12.0.0]
        at org.opensearch.security.filter.SecurityFilter.apply(SecurityFilter.java:165) [opensearch-security-2.12.0.0.jar:2.12.0.0]
        at org.opensearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:216) [opensearch-2.12.0.jar:2.12.0]
        at org.opensearch.action.support.TransportAction.execute(TransportAction.java:188) [opensearch-2.12.0.jar:2.12.0]
        at org.opensearch.action.support.TransportAction.execute(TransportAction.java:107) [opensearch-2.12.0.jar:2.12.0]
        at org.opensearch.client.node.NodeClient.executeLocally(NodeClient.java:110) [opensearch-2.12.0.jar:2.12.0]
        at org.opensearch.client.node.NodeClient.doExecute(NodeClient.java:97) [opensearch-2.12.0.jar:2.12.0]
        at org.opensearch.client.support.AbstractClient.execute(AbstractClient.java:476) [opensearch-2.12.0.jar:2.12.0]
        at org.opensearch.sql.spark.rest.RestAsyncQueryManagementAction.lambda$executePostRequest$1(RestAsyncQueryManagementAction.java:126) [spark-2.12.0.0-SNAPSHOT.jar:?]
        at org.opensearch.sql.datasources.utils.Scheduler.lambda$withCurrentContext$0(Scheduler.java:30) [datasources-2.12.0.0-SNAPSHOT.jar:?]
        at org.opensearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:854) [opensearch-2.12.0.jar:2.12.0]
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) [?:?]
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) [?:?]
        at java.base/java.lang.Thread.run(Thread.java:1583) [?:?]
[2024-02-06T21:09:22,689][INFO ][o.o.j.s.JobSweeper       ] [node-1] Running full sweep

What solution would you like?

Instead of doing validation while reading the configuration. Is it possible to block the empty bracket and perform the validation beforehand?