ES|QL: Check for unsupported fields in FUSE #135530
                
     Merged
            
            
          
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Tracked in #123389
Since FUSE is using STATS BY underneath to merge rows together, we need to make sure all the input columns will be supported by STATS.
As a follow up, we can look into alternatives, so FUSE can output unsupported columns which by convention only return null values (similar to FORK).