[PLAT-12988] Set bugsnag.span.category to 'custom' for custom spans

[kstenerud]

Custom spans now set attribute bugsnag.span.category to custom.

e2e tests have been updated to check for this.

[github-actions]

BugsnagPerformance.framework binary size increased by 232 bytes from 518,216 to 518,448

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.3%    +292  +0.3%    +292    __TEXT,__text
  +0.1%    +112  +0.1%    +112    String Table
  +0.1%    +112  +0.1%    +112    Symbol Table
  +0.5%     +56  +0.5%     +56    __TEXT,__gcc_except_tab
  +0.4%     +32  +0.4%     +32    __DATA_CONST,__cfstring
  +0.5%     +24  +0.5%     +24    __TEXT,__unwind_info
  +0.7%      +8  +0.7%      +8    Function Start Addresses
  +0.1%      +7  +0.1%      +7    __TEXT,__cstring
  -1.4%     -32  -1.4%     -32    [__DATA_CONST]
  [ = ]       0  -3.8%    -232    [__LINKEDIT]
  -0.8%    -379  -0.8%    -379    [__TEXT]
  +0.0%    +232  [ = ]       0    TOTAL

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