ordered_json doesn't accept keys of types other than string_t (e.g., string_view)

[falbrechtskirchinger]

Description

When adding support for string_view, we overlooked that ordered_map used by ordered_json would also need an update to properly support types other than object_t::key_type/string_t.

Reproduction steps

See discussion #3556.

Expected vs. actual results

See discussion #3556.

Minimal code example

No response

Error messages

No response

Compiler and operating system

any

Library version

develop

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.