Added asNumber to OrderByColumnSpec

This commit make dimension can be sorted numerically(only "long" here) besides lexicographically. e.g
```
query:
...
"dimensions": [
	"id",
],
"limitSpec": {
	"type": "default",
	"limit": 20,
	"columns": [
		{
			"asNumber": true,
			"dimension": "id",
			"direction": "ASCENDING"
		}
	]
},
...

result:
id	name	score
0	alias	90
1	bob		75
2	joe		60
11	flow	88
14	kevin	54
```
Please have an explicit type parameter here if at all possible, rather than Object