Distinguish between exact and case insensitive matching JSON keys in json_structure

[lnkuiper]

The json_structure function now ignores errors again (like it used to), so these are allowed:

select json_structure('{"a": 1, "a": 1}');
-- {"a":"UBIGINT"}
select json_structure('{"a": 1, "A": 1}');
-- {"a":"UBIGINT","A":"UBIGINT"}

Fixes #11886

[hannes]

Can you check the failing tests please?

[Mytherin]

Thanks!