Introduces a new 'unlimited_user' flag for users. When set to true, the user is exempt from the concurrent IP limit enforcement by `limit.sh`.
- `core/scripts/hysteria2/add_user.py`:
- Adds an optional `unlimited_user` argument (defaults to false).
- Stores the `unlimited_user` boolean in the `users.json` file when adding a user.
- `core/scripts/hysteria2/limit.sh`:
- Modifies `check_ip_limit` function to first check the `unlimited_user` flag for the connecting user in `users.json`.
- If the flag is true, the script bypasses the IP limit check for that user.
- Uses `jq` for reliable parsing and includes a `grep` fallback for systems without `jq`.
- Existing users without the flag are treated as limited (default behavior).
