Implement a `swept_center_frequency` mode for the HackRF One

[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 HackRF One 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.