Bypass FFT calculation on SDR

[Debupt]

Feature description
I have already calculated the FFT for the data and sent it via socket to SDRPP. Now, I want to bypass the current FFT calculation and directly display the raw data (which is already the spectrum results), but reuse waterfall and other related features

[AlexandreRouma]

planned since it's needed for the SDR++ server feature to work more efficiently