[Backport 2.19-dev] Allow equal expression as a function argument

[opensearch-trigger-bot]

Backport 9f2b19a from #4001.

[yuancu]

There's a failed test:

org.opensearch.sql.calcite.tpch.CalcitePPLTpchIT > testQ7 FAILED
    java.net.SocketTimeoutException: 60 000 milliseconds timeout on connection http-outgoing-469 [ACTIVE]
        at __randomizedtesting.SeedInfo.seed([533FB44206321030:8473C8A764182F94]:0)
        at org.opensearch.client.RestClient.extractAndWrapCause(RestClient.java:1230)
        at org.opensearch.client.RestClient.performRequest(RestClient.java:358)
        at org.opensearch.client.RestClient.performRequest(RestClient.java:361)
        at org.opensearch.client.RestClient.performRequest(RestClient.java:346)
        at org.opensearch.sql.ppl.PPLIntegTestCase.executeQueryToString(PPLIntegTestCase.java:48)
        at org.opensearch.sql.ppl.PPLIntegTestCase.executeQuery(PPLIntegTestCase.java:44)
        at org.opensearch.sql.calcite.tpch.CalcitePPLTpchIT.testQ7(CalcitePPLTpchIT.java:178)

        Caused by:
        java.net.SocketTimeoutException: 60 000 milliseconds timeout on connection http-outgoing-469 [ACTIVE]
            at org.apache.http.nio.protocol.HttpAsyncRequestExecutor.timeout(HttpAsyncRequestExecutor.java:387)
            at org.apache.http.impl.nio.client.InternalIODispatch.onTimeout(InternalIODispatch.java:98)
            at org.apache.http.impl.nio.client.InternalIODispatch.onTimeout(InternalIODispatch.java:40)
            at org.apache.http.impl.nio.reactor.AbstractIODispatch.timeout(AbstractIODispatch.java:175)
            at org.apache.http.impl.nio.reactor.BaseIOReactor.sessionTimedOut(BaseIOReactor.java:261)
            at org.apache.http.impl.nio.reactor.AbstractIOReactor.timeoutCheck(AbstractIOReactor.java:506)
            at org.apache.http.impl.nio.reactor.BaseIOReactor.validate(BaseIOReactor.java:211)
            at org.apache.http.impl.nio.reactor.AbstractIOReactor.execute(AbstractIOReactor.java:280)
            at org.apache.http.impl.nio.reactor.BaseIOReactor.execute(BaseIOReactor.java:104)
            at org.apache.http.impl.nio.reactor.AbstractMultiworkerIOReactor$Worker.run(AbstractMultiworkerIOReactor.java:591)
            at java.base/java.lang.Thread.run(Thread.java:829)

Can this be safely viewed as flaky? @LantaoJin

[LantaoJin]

Yes. The flaky will be fixed in my other PR