Add core:hw to SigMF metadata files

[argilo]

This is a follow-up to #1182.

As noted in #1182 (comment), it would be useful to store input device metadata in the core:hw field, which the SigMF spec defines as:

A text description of the hardware used to make the Recording.

Unfortunately, the gr-osmosdr API does not appear to provide any means to fetch metadata about the device associated with a source block. Metadata about all scanned devices can be retrieved with osmosdr::device::find(), but there is no guarantee that the device selected by the device string matches one of the scanned devices. So I think the best we can do for now is write out the current device string, e.g. "core:hw": "airspyhf=0".

/cc @Niautanor

[willcode]

Too bad there's no way of declaring something purely informational. I can see someone parsing this string, since it means something to osmo. Maybe add a prefix? Something like
osmo://airspyhf=0
Not sure I really like that exact syntax though because we could potentially use a URI for something like ZMQ later. So,
osmo: airspyhf=0 would be a less formal way of saying the same thing. Just some ideas.

[argilo]

The spec sounds like it's meant to be human-readable. How about "gr-osmosdr device specification: airspyhf=0"?

[willcode]

A little less verbose would be nice, in case it's used in a title bar or something, but yah, something that doesn't look parseable.

[argilo]

A little less verbose would be nice

OK, I changed the format to OsmoSDR: airspyhf=0.