Support uppercase user defined variables

[harshit-gangal]

This handled uppercase UDV which causing issues of missing bindvars from the vttablet
#6218

Similar to mysql behaviour.

mysql > select @myvalue;
+----------+
| @myvalue |
+----------+
| NULL     |
+----------+
1 row in set (0.00 sec)

mysql > set @MYVALUE = 42;
Query OK, 0 rows affected (0.00 sec)

mysql > select @myvalue;
+----------+
| @myvalue |
+----------+
|       42 |
+----------+
1 row in set (0.00 sec)

Signed-off-by: Harshit Gangal harshit@planetscale.com