fix: avoid duplicated apikey from data import

[JimMoen]

Fixes EMQX-11209

Mnesia uses the first field of the record as the key to identify a record.

In this case:
Table emqx_app's key is name, and it is randomly generated and has no association with api_key.
So when exporting/importing on two different clusters, there will be two records in mnesia with the same api_key but different name, which will cause a case_caluse when node restart after importing.

Summary

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This pull request improves the management API key feature by refactoring the creation and authentication logic, adding app key support for the tests, and documenting the dashboard API key function. It affects the files emqx_mgmt_auth.erl, emqx_common_test_http.erl, and emqx_mgmt_api_api_keys.erl.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Added property-based tests for code which performs user input validation
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update