[PLAT-9970] Include API key when serializing events to JSON

[yousif-bugsnag]

Goal

Include the event's API key when serializing/deserilaising events to/from JSON.

Fixes an issue where an overridden API key is not preserved if the event is persisted to disk (for example if the event is marked as unhandled or if there is no network connection)

Changeset

Added API key to the initWithJson and toJsonWithRedactedKeys methods on BugsnagEvent

Testing

Updated the JSON serialization unit test for BugsnagEvent to include an API key

[github-actions]

Bugsnag.framework binary size did not change - 707,048 bytes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%    +128  +0.1%    +128    __TEXT,__text
  -0.5%    -128  -0.5%    -128    [__TEXT]
  [ = ]       0  [ = ]       0    TOTAL

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