[NEW CONTENT TYPE REQUEST] GNU radio save files

**What type of file would you like magika to detect?**
GNU radio files

**What software can create/open these files?**
https://www.gnuradio.org/

**Where can these files be found?**
See the tutorials: https://wiki.gnuradio.org/index.php?title=Your_First_Flowgraph

**If possible, please provide a specification for this file type.**
https://wiki.gnuradio.org/index.php/File_Source

**Additional context**
Very useful software.

Start by reviewing the GNU Radio site, the Your First Flowgraph tutorial, and the File Source documentation linked in the issue to identify the save-file format and representative samples. Then inspect Magika's existing content-type detection workflow and tests to determine where a new type belongs. Done means GNU Radio save files are reliably detected and covered by tests.

Written by the indexing model from the issue text.