### What changes were proposed in this pull request?
At the moment we do not have any function to compute length of JSON array directly.
I propose a `json_array_length` function which will return the length of the outermost JSON array.
- This function will return length of the outermost JSON array, if JSON array is valid.
```
scala> spark.sql("select json_array_length('[1,2,3,[33,44],{\"key\":[2,3,4]}]')").show
+--------------------------------------------------+
|json_array_length([1,2,3,[33,44],{"key":[2,3,4]}])|
+--------------------------------------------------+
| 5|
+--------------------------------------------------+
scala> spark.sql("select json_array_length('[[1],[2,3]]')").show
+------------------------------+
|json_array_length([[1],[2,3]])|
+------------------------------+
| 2|
+------------------------------+
```
- In case of any other valid JSON string, invalid JSON string or null array or `NULL` input , `NULL` will be returned.
```
scala> spark.sql("select json_array_length('')").show
+-------------------+
|json_array_length()|
+-------------------+
| null|
+-------------------+
```
### Why are the changes needed?
- As mentioned in JIRA, this function is supported by presto, postgreSQL, redshift, SQLite, MySQL, MariaDB, IBM DB2.
- for better user experience and ease of use.
```
Performance Result for Json array - [1, 2, 3, 4]
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_array_length 7728 7762 53 1.3 772.8 1.0X
size+from_json 12739 12895 199 0.8 1273.9 0.6X
```
### Does this PR introduce any user-facing change?
Yes, now users can get length of a json array by using `json_array_length`.
### How was this patch tested?
Added UT.
Closes#27759 from iRakson/jsonArrayLength.
Authored-by: iRakson <raksonrakesh@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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