#1286. Support sample node in explain statement

After the fix:
```
presto:default> explain select * from nation tablesample system(50);
                                                                    Query Plan                                                         
---------------------------------------------------------------------------------------------------------------------------------------
 - Output[n_nationkey, n_name, n_regionkey, n_comment]                                                                                 
         n_nationkey := n                                                                                                              
         n_name := n_1                                                                                                                 
         n_regionkey := n_2                                                                                                            
         n_comment := n_3                                                                                                              
     - Sample[0.5] => [n:bigint, n_1:varchar, n_2:bigint, n_3:varchar]                                                                 
         - TableScan[hive:default:nation, original constraint=true] => [n:bigint, n_1:varchar, n_2:bigint, n_3:varchar]                
                 n := HiveColumnHandle{clientId=hive, name=n_nationkey, ordinalPosition=0, hiveType=INT, hiveColumnIndex=0, partitionKe
                 n_1 := HiveColumnHandle{clientId=hive, name=n_name, ordinalPosition=1, hiveType=STRING, hiveColumnIndex=1, partitionKe
                 n_2 := HiveColumnHandle{clientId=hive, name=n_regionkey, ordinalPosition=2, hiveType=INT, hiveColumnIndex=2, partition
                 n_3 := HiveColumnHandle{clientId=hive, name=n_comment, ordinalPosition=3, hiveType=STRING, hiveColumnIndex=3, partitio
                                                                                                                                       
(1 row)
```
Are there any explain unit tests I can add this to? Couldn't find any.