Simplify ApiKey entity by exposing key as a readonly prop

[acelaya]

Part of #2193

Remove toString and __toString methods from ApiKey entity, and instead expose key as a public readonly property.

This will simplify finding all usages of key in future, when the key becomes a hash of the actual key.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.53%. Comparing base (3085fa7) to head (79c5418).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #2248      +/-   ##
=============================================
- Coverage      93.54%   93.53%   -0.01%     
+ Complexity      1613     1611       -2     
=============================================
  Files            270      270              
  Lines           5635     5631       -4     
=============================================
- Hits            5271     5267       -4     
  Misses           364      364              

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