ksqlDb Java Client should support EXPLAIN statements

[wicknicks]

Is your feature request related to a problem? Please describe.
We are using the java client to interact with ksqldb clusters. but the client is unable to execute EXPLAIN statements, which is useful to look at output schemas and execution plan of queries.

Describe the solution you'd like
The Java client should add support for QueryDescription entities which contains the bulk of the response for this statement.

Describe alternatives you've considered
Would be nice to have a "low level" method that returns the raw response for any generic ksqldb statement. This way, users don't have to wait for changes in java client to unblock them.

Additional context
Error message when trying to run explain statements:

java.util.concurrent.ExecutionException: io.confluent.ksql.api.client.exception.KsqlClientException: The executeStatement() method is only for 'CREATE', 'CREATE ... AS SELECT', 'DROP', 'TERMINATE', and 'INSERT INTO ... AS SELECT' statements. The client does not currently support 'EXPLAIN <QUERY_ID>' statements.
	at java.base/java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:395)
	at java.base/java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1999)

Caused by: io.confluent.ksql.api.client.exception.KsqlClientException: The executeStatement() method is only for 'CREATE', 'CREATE ... AS SELECT', 'DROP', 'TERMINATE', and 'INSERT INTO ... AS SELECT' statements. The client does not currently support 'EXPLAIN <QUERY_ID>' statements.
	at io.confluent.ksql.api.client.impl.DdlDmlResponseHandlers.handleUnexpectedEntity(DdlDmlResponseHandlers.java:97)
	at io.confluent.ksql.api.client.impl.DdlDmlResponseHandlers.handleExecuteStatementResponse(DdlDmlResponseHandlers.java:41)
	at io.confluent.ksql.api.client.impl.ClientImpl.lambda$handleSingleEntityResponse$22(ClientImpl.java:579)
	at io.vertx.core.http.impl.HttpClientResponseImpl$BodyHandler.notifyHandler(HttpClientResponseImpl.java:292)