Adding support for up/down converters

[ohault]

SoapySDR provides a great neutral support of SDR vendors and platforms.

In the same idea, support for up/down converters could be interesting.

I could be for both fixed LNB LO frequency and also configurable up/down converter devices.

e.g.

• https://www.rtl-sdr.com/testing-the-electrosense-up-downconverter-expansion-board-for-0-6-ghz/

A way to reflect this parameter value on the whole stack would be also required - https://github.com/pothosware/SoapyRTLSDR/issues/75