Implement a `swept_center_frequency` mode for RTL-SDR devices

[jcfitzpatrick12]

Description

I was reading the docs for the Soapy blocks, and noticed that they accept messages on the cmd message port. If this is true for the RTL-SDR source, we should be able to implement a swept_center_frequency mode analogous to those already implemented for the USRP b200mini and SDRplay receivers.

As part of this issue, we need to explore if the existing functionality of the spectre_sweep_driver OOT block suffices to send retune commands to the Soapy source message port. Additionally, we may need to contribute to the Soapy plugin so that the block can emit a stream tag.

See for reference:

• https://github.com/pavels/spektrum
• https://www.rtl-sdr.com/khanfar-spectrum-analyzer-signal-analysis-tools-for-the-rtl-sdr/
• ...