add a new API to report report feature states

[ffromani]

When inspecting features, the current API reports only if each feature is active. Client applications, especially those consuming the output of ethtool -k [iface] also want to know if a feature can actually be enabled or not. Examples:

• https://github.com/jaypipes/ghw/blob/main/README.md#network -https://github.com/jaypipes/ghw/blob/main/pkg/net/net_linux.go#L161

This information is already available but is not exposed to the API. So, add a new API to export all the flags. We add a new API instead of changing the existing one to avoid breaking the backward compatibility.

Closes: #50 Signed-off-by: Francesco Romani [email protected]