[PLAT-11642] BugsnagSessionTracker race condition

[kstenerud]

Goal

BugsnagSessionTracker.extraRuntimeInfo was not protected from asynchronous access, potentially crashing RN apps.

Changeset

Added @synchronized to protect the dictionary.

Testing

Added unit test.

[github-actions]

Bugsnag.framework binary size increased by 56 bytes from 713,208 to 713,264

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%    +120  +0.1%    +120    __TEXT,__text
  +0.0%     +32  +0.0%     +32    Symbol Table
  +1.0%     +32  +1.0%     +32    __TEXT,__gcc_except_tab
  +0.0%     +24  +0.0%     +24    String Table
  +0.4%     +12  +0.4%     +12    __TEXT,__unwind_info
  [ = ]       0  -0.7%     -56    [__LINKEDIT]
  -0.9%    -164  -0.9%    -164    [__TEXT]
  +0.0%     +56  [ = ]       0    TOTAL

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