Use unique callsigns for each flight

[zhexu14]

This PR partially addresses #1561 by automatically generating unique callsigns for each flight.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.82%. Comparing base (38c3cdb) to head (37546d4).
Report is 17 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3445      +/-   ##
===========================================
- Coverage    47.65%   46.82%   -0.83%     
===========================================
  Files          476       65     -411     
  Lines        27112      865   -26247     
  Branches      4607      145    -4462     
===========================================
- Hits         12919      405   -12514     
+ Misses       14170      460   -13710     
+ Partials        23        0      -23     

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