support nlmon link type

[cathay4t]

Introduce nlmon (Netlink monitor) interface support for both creation
and query. Usage:

ip link add name NAME type nlmon

Integration test cases included.

[gemini-code-assist]

Code Review

This pull request adds support for the nlmon link type, including the necessary logic in LinkAddCommand and a new test suite in src/ip/link/tests/nlmon.rs. The reviewer suggested improving the with_nlmon_iface test helper by using std::panic::resume_unwind to preserve panic information during test failures and making the interface cleanup step more robust.

[gemini-code-assist]

The current implementation of with_nlmon_iface uses assert!(result.is_ok()) after catching a panic. This is suboptimal because it discards the original panic message and stack trace, making it difficult to debug test failures. Using std::panic::resume_unwind is preferred as it preserves the original panic information. Additionally, the cleanup step should ideally be robust against failures (e.g., if the interface was already removed by the test itself).

    let result = std::panic::catch_unwind(test);

    // clean up
    let _ = exec_cmd(&["ip", "link", "del", name]);

    if let Err(e) = result {
        std::panic::resume_unwind(e);
    }