ESQL: Add syntax for FIRST and LAST

```
| STATS FIRST(v BY @timestamp) BY hostname
```

Related to #108385

Author: nik9000

x-pack/plugin/esql/src/main/antlr/parser/Expression.g4

@@ -49,7 +49,8 @@ primaryExpression
     ;
 
 functionExpression
-    : functionName LP (ASTERISK | (booleanExpression (COMMA booleanExpression)* (COMMA mapExpression)?))? RP
+    : functionName LP (ASTERISK | (booleanExpression (COMMA booleanExpression)* (COMMA mapExpression)?))? RP  #functionStandard
+    | (FIRST | LAST) LP value=booleanExpression BY by=booleanExpression RP                                             #functionFirstLast
     ;
 
 functionName