[SPARK-21901][SS] Define toString for StateOperatorProgress

## What changes were proposed in this pull request?

Just `StateOperatorProgress.toString` + few formatting fixes

## How was this patch tested?

Local build. Waiting for OK from Jenkins.

Author: Jacek Laskowski <jacek@japila.pl>

Closes #19112 from jaceklaskowski/SPARK-21901-StateOperatorProgress-toString.

sql/core/src/main/scala/org/apache/spark/sql/streaming/progress.scala

@@ -55,6 +55,8 @@ class StateOperatorProgress private[sql](
     ("numRowsUpdated" -> JInt(numRowsUpdated)) ~
     ("memoryUsedBytes" -> JInt(memoryUsedBytes))
   }
+
+  override def toString: String = prettyJson
 }
 
 /**