1. move `group_concat` from string-functions to aggregate-functions.
2. add `json_array`/`json_object`/`json_quote` to sidebar file.
3. move `json_array`/`json_object`/`json_quote`/`get_json_double`/`get_json_int`/`get_json_string` to json-functions.
4. change `group_concat` document to uppercase
For #4674
This is a udaf for approximate topn using Space-Saving algorithm. At present, we can only calculate
the frequent items and their frequencies in a certain column, based on which we can implement similar
topN functions supported by Kylin in the future.
I have also added a test to calculate the accuracy of this algorithm. The following is a rough running result.
The total amount of data is 1 million lines and follows the Zipfian distribution, where Element Cardinality
represents the data cardinality, 20X, 50X.. The value representing space_expand_rate is 20,50, which is
used to set the counter number in the space-saving algorithm
```
zf exponent = 0.5
Element cardinality 20X 50X 100X
1000 100% 100% 100%
10000 100% 100% 100%
100000 100% 100% 100%
500000 94% 98% 99%
zf exponent = 0.6，1
Element cardinality 20X 50X 100X
1000 100% 100% 100%
10000 100% 100% 100%
100000 100% 100% 100%
500000 100% 100% 100%
```
