scos-sensor actions should properly document SigMF extensions.

[jhazentia]

https://github.com/gnuradio/SigMF/blob/master/sigmf-spec.md#the-extensions-field

sigmf/SigMF#105

[aromanielloNTIA]

This has been addressed as part of #244

Note that the extensions array is still not dynamically populated based on which extensions have actually been used in a given metadata file, but the format is now correct/up-to-date with SigMF