*: add builtin function json_storage_size

[stoksc]

What problem does this PR solve?

close #14488

What is changed and how it works?

Add a builtin function for json_storage_size. My only concern is if the length of the marshaled json is correct as the storage size.

Tests

Added unit tests, as well as this local manual test:

tidb> CREATE TABLE jtable (jcol JSON);
Query OK, 0 rows affected (0.01 sec)

tidb> INSERT INTO jtable VALUES  ('{"a": 1000, "b": "wxyz", "c": "[1, 3, 5, 7]"}');
Query OK, 1 row affected (0.01 sec)

tidb> select jcol, JSON_STORAGE_SIZE(jcol) as size from jtable;
+-----------------------------------------------+------+
| jcol                                          | size |
+-----------------------------------------------+------+
| {"a": 1000, "b": "wxyz", "c": "[1, 3, 5, 7]"} |   45 |
+-----------------------------------------------+------+
1 row in set (0.00 sec)

tidb> UPDATE jtable SET jcol = 
    ->     JSON_SET(jcol, "$.b", "a");
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

tidb> select jcol, JSON_STORAGE_SIZE(jcol) as size from jtable;
+--------------------------------------------+------+
| jcol                                       | size |
+--------------------------------------------+------+
| {"a": 1000, "b": "a", "c": "[1, 3, 5, 7]"} |   42 |
+--------------------------------------------+------+
1 row in set (0.00 sec)

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[qw4990]

@stoksc Thanks for your contribution, we will review this PR soon~

[wjhuang2016]

Thanks, could you add some tests like https://dev.mysql.com/doc/refman/8.0/en/json-utility-functions.html#function_json-storage-size?
That is, check the result of json_storage_size after partial updating it.

[stoksc]

Added it to the above tests section.

[wjhuang2016]

LGTM

[zz-jason]

LGTM

[sre-bot]

Your auto merge job has been accepted, waiting for 14920

[sre-bot]

/run-all-tests