Add a SpectrumPhy method to early discard signals being received

This MR adds a hook into the StartTx method of {Single,Multi}ModelSpectrumChannel to allow PHYs to early discard signals being received.

Such a hook is only used (at the moment) by the SpectrumWifiPhy to early discard PPDUs whose TX band (including guard bands) does not overlap the band of the operating channel of the receiver. This allows us to avoid useless computation of the attenuation due to the propagation and of the receive power when the PPDU is not going to be received. This is especially relevant for 11be multi-link devices operating on multiple channels/bands.