Add PlutoSDR support.

[AMDBartek]

This simple pull-requests adds support for the ADALM-PLUTO and Pluto-like devices. I personally tested it on a Pluto+ and it seems to work fine with the example images in the repo and my own as well:

~/spectrum_painter master*
.venv ❯ spectrum_painter -s 1000000 -l 0.01 -o smiley.cs16 --format plutosdr ./examples/smiley.png

~/spectrum_painter master*
.venv ❯ tx_sdr -f 420.69M -s 1000000 -F CS16 -B 1M smiley.cs16 -g 70

Result: