The future implementation of Multi SF support with the LR2021

[serialrf433]

The LR2021 added support for multi SF. This would add nice new capabilities. For example many communities are changing from Long_Fast to something with more capacity. The LR2021 can support Long_Fast and the other setting at the same time.

The Datasheet of the LR2021 is out now. Current version of the datasheet is v1.1. https://www.semtech.com/products/wireless-rf/lora-plus/lr2021
In the firmware things like this have to be implemented at best possible way:
It is essential to ensure that the main detector is programmed to receive the highest spreading factor that the receiver is expected to handle.

These are the relevant things that could be interesting to talk about:

 The following rules apply (also apply to Multi-CAD):
* Up to 4 Spreading Factors (SF) can be detected in parallel (main SF + 3 side detectors).
* The main SF (configured with SetLoraModulationParams) must be the smallest SF in normal Rx operation and the largest SF for CAD operation.
* The header type is fixed by the main SF.
* If the BW is greater than 500kHz, only a maximum of two side detectors are allowed.
* If the main SF is 10, 11, or 12, only one side detector is allowed (maximum two SFs can be detected simultaneously).
* All SFs must be different; it is not possible to have two of the same SF in parallel.
* The difference between the Max SF and Min SF must be less than or equal to 4.
* After a successful reception, it is possible to know which SF was demodulated with GetLoraPacketStatus (see detector(2:0)).
* Calling SetLoraModulationParams disables all side detectors, and the developer must re-configure them.

RadioLib is getting LR2021 support in version 8.0.0 jgromes/RadioLib#1457

@GUVWAF i am interested in your opinion how the multi SF could be implemented at best.