Add startSpan method to crosstalk API

[yousif-bugsnag]

Goal

Add support for starting spans via the crosstalk API

Testing

Added a unit test for the new crosstalk API method and tested manually via React Native Performance

[github-actions]

BugsnagPerformance.framework binary size increased by 560 bytes from 519,736 to 520,296

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.5%    +532  +0.5%    +532    __TEXT,__text
  +0.3%    +304  +0.3%    +304    Symbol Table
  +0.2%    +256  +0.2%    +256    String Table
  +0.7%    +120  +0.7%    +120    __DATA,__objc_const
  +0.7%    +106  +0.7%    +106    __TEXT,__objc_methname
  +2.4%     +71  +2.4%     +71    __TEXT,__objc_methtype
  +0.5%     +64  +0.5%     +64    __TEXT,__objc_stubs
  +0.3%     +40  +0.3%     +40    __TEXT,__gcc_except_tab
  +0.4%     +32  +0.4%     +32    __DATA_CONST,__cfstring
  +0.7%     +24  +0.7%     +24    __DATA,__objc_selrefs
  +0.5%     +24  +0.5%     +24    __TEXT,__unwind_info
  +0.2%     +12  +0.2%     +12    __TEXT,__cstring
  +0.9%      +4  +0.9%      +4    __DATA,__objc_ivar
  -1.4%     -32  -1.4%     -32    [__DATA_CONST]
  -2.1%    -148  -2.2%    -148    [__DATA]
  [ = ]       0 -12.3%    -560    [__LINKEDIT]
  -1.8%    -849  -1.8%    -849    [__TEXT]
  +0.1%    +560  [ = ]       0    TOTAL

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