Add a way to notify the current connection status to higher network layers

[SamClercky]

At the moment there is no way of indicating whether or not the link layer is active. This can become interesting when integrating with Embassy as it would prevent packets being sent while the underlying layers are still setting up (ie. initial phases of TSCH).

This would probably require some changes in:

• The Radio trait, as it would need a way to signal that the radio is ready to transmit (also what about sleep states?)
• The Driver trait, as it would need to propagate the current state to higher layers

Adding this issue here, as it would be a major breaking change and more expensive to change later on.