GFSK support for SX1231/RFM69 and SX127x/RFM9x

[LorbusChris]

Is your feature request related to a problem? Please describe.
I'd like to use RadioLib to receive GFSK modulated messages with RFM69 and RFM95 modules.
Per its spec sheets, the modules should support GFSK.

https://cdn.sparkfun.com/datasheets/Wireless/General/RFM69HCW-V1.1.pdf
https://cdn.sparkfun.com/assets/a/e/7/e/b/RFM95_96_97_98W.pdf

Describe the solution you'd like
Add support for GFSK on SX1231/RFM69 and SX127x/RFM9x modules

Describe alternatives you've considered
The RadioHead library ships support for GFSK on RFM69 modules:

https://github.com/adafruit/RadioHead/blob/master/RH_RF69.h (fork)

Additional context

[jgromes]

This is already implemented - for SX127x, see the FSK example, you can set the Gaussian filter by calling setDataShaping (as also shown in that example).

For RF69, again, setDataShaping can be used to configure the Gaussian filter BT product.

[LorbusChris]

Awesome, thanks!