Add private app authenticate API for bibxml

`resolution_fixed` `type_enhancement`   |    by kesara@staff.ietf.org

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Create a private app authentication API for bibxml similar to Author Tools (#3396).

Proposed API endpoint: `/api/appauth/bibxml`

Returns following JSON output when successful:
```
{"success": true}
```

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_Issue migrated from trac:3480 at 2022-03-04 09:21:24 +0000_