[BugFix] try to parse it when casting string to json

[mofeiatwork]

What type of PR is this：

• bug
• feature
• enhancement
• refactor
• others

Which issues of this PR fixes ：

Fixes #8127

Problem Summary(Required) ：

Before

mysql> select cast('1'as json);
-------------------------------------------
"1"

After

mysql> select cast('1' as json);
-------------------------------------------
1

This change would make the implicit casting behavior more user-friendly, in most cases user do not need to manually call the parse_json function.

E.g. Before this PR, user should write select json_query(parse_json('{"a": 1}), 'a') to get a valid JSON, but not the select json_query('{"a": 1}', 'a'). Cause the second case insert an implicit cast(varchar as json) , which would tread the varchar as a json subtype string, but not parse it as a json object.

[mofeiatwork]

@Mergifyio backport branch-2.3

[mergify]

backport branch-2.3

🟠 Waiting for conditions to match

• merged [:pushpin: backport requirement]

[mergify]

backport branch-2.3

✅ Backports have been created

• #7863 [BugFix] try to parse it when casting string to json (backport #7835) has been created for branch branch-2.3

[mofeiatwork]

@Mergifyio backport branch-2.2

[mofeiatwork]

@Mergifyio backport branch-2.2

[mergify]

backport branch-2.2

✅ Backports have been created

• #8003 [BugFix] try to parse it when casting string to json (backport #7835) has been created for branch branch-2.2