### What changes were proposed in this pull request?
A new function `json_object_keys` is proposed in this PR. This function will return all the keys of the outmost json object. It takes Json Object as an argument.
- If invalid json expression is given, `NULL` will be returned.
- If an empty string or json array is given, `NULL` will be returned.
- If valid json object is given, all the keys of the outmost object will be returned as an array.
- For empty json object, empty array is returned.
We can also get JSON object keys using `map_keys+from_json`. But `json_object_keys` is more efficient.
```
Performance result for json_object = {"a":[1,2,3,4,5], "b":[2,4,5,12333321]}
Intel(R) Core(TM) i7-9750H CPU 2.60GHz
JSON functions: Best Time(ms) Avg Time(ms) Stdev(ms) Rate(M/s) Per Row(ns) Relative
------------------------------------------------------------------------------------------------------------------------
json_object_keys 11666 12361 673 0.9 1166.6 1.0X
from_json+map_keys 15309 15973 701 0.7 1530.9 0.8X
```
### Why are the changes needed?
This function will help naive users in directly extracting the keys from json string and its fairly intuitive as well. Also its extends the functionality of spark-sql for json strings.
Some of the most popular DBMSs supports this function.
- PostgreSQL
- MySQL
- MariaDB
### Does this PR introduce any user-facing change?
Yes. Now users can extract keys of json objects using this function.
### How was this patch tested?
UTs added.
Closes#27836 from iRakson/jsonKeys.
Authored-by: iRakson <raksonrakesh@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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