Add functionality to reset or clear usage statistics for access keys

[freddicruger]

Currently, the Outline VPN API does not provide a way to reset or clear the usage statistics (e.g., used bytes) for access keys. While it is possible to set or remove data limits for keys, there is no option to manually reset the counters for traffic usage.

This feature would be useful in scenarios where administrators need to:

Periodically reset usage statistics for reporting purposes.
Reuse existing keys without creating new ones.
Provide users with a "fresh start" for their traffic usage.
Proposed Solution:
Add a new API endpoint (and corresponding method in the outline_vpn Python library) to reset the usage statistics for a specific key. For example:

Endpoint: POST /access-keys/:id/reset-usage
Behavior: Reset the used_bytes counter for the specified key to 0.
Alternatively, if modifying the server is not feasible, consider documenting a recommended approach for achieving this (e.g., via database manipulation).

Workaround:
Currently, the only workaround is to delete the key and create a new one, which is not ideal for all use cases.

Thank you for considering this feature request! Please let me know if you need additional details or clarification.

[sbruens]

Thanks for filing this feature request. We have an existing issue to add support for this in the Manager Jigsaw-Code/outline-apps#1860, but we can consider adding it only for the server. I'll leave this open as its counterpart.