implement heavy lifter SPA

[NickAragua]

Now that mechs can pick up cargo, it's very easy to implement the "heavy lifter" SPA.

Also includes some null safety for an entity's carried cargo collection.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.01%. Comparing base (d349884) to head (37a465c).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5839   +/-   ##
=========================================
  Coverage     29.01%   29.01%           
- Complexity    13923    13928    +5     
=========================================
  Files          2511     2511           
  Lines        267117   267123    +6     
  Branches      47784    47787    +3     
=========================================
+ Hits          77504    77510    +6     
- Misses       185661   185666    +5     
+ Partials       3952     3947    -5     

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