ES|QL: Change FUSE KEY BY to receive a list of qualifiedName (elastic#139071)

Author: ioanatia

docs/changelog/139071.yaml

@@ -0,0 +1,5 @@
+pr: 139071
+summary: Change FUSE KEY BY to receive a list of `qualifiedName`
+area: ES|QL
+type: bug
+issues: []

x-pack/plugin/esql/src/main/antlr/EsqlBaseParser.g4

@@ -347,11 +347,15 @@ fuseCommand
 
 fuseConfiguration
     : SCORE BY score=qualifiedName
-    | KEY BY key=fields
+    | KEY BY key=fuseKeyByFields
     | GROUP BY group=qualifiedName
     | WITH options=mapExpression
     ;
 
+fuseKeyByFields
+   : qualifiedName (COMMA qualifiedName)*
+   ;
+
 //
 // In development
 //