`GREATEST` and `LEAST` functions should return decimal type when args are decimal types

Allowing decimal type arguments (#10562) was implemented in dolthub/go-mysql-server#3447 but the return type is a float64 when it should be a decimal of the highest precision.
```sql
mysql> create table t(a decimal(6, 2), b decimal(8, 5), c decimal(5, 1));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into t values (2.75, 8.8, 3.1);
Query OK, 1 row affected (0.01 sec)

mysql> select greatest(a, b, c), least(a, b, c) from t;
+-------------------+----------------+
| greatest(a, b, c) | least(a, b, c) |
+-------------------+----------------+
| 8.80000 | 2.75000 |
+-------------------+----------------+
1 row in set (0.00 sec)
```

We also need to consider scale and when the argument types are mixed.