[PLAT-31288] Split out ending a span and closing/sending for processing

[kstenerud]

Goal

The RN library needs to be able to set the end time multiple times, so we need to split out:

• ending a span (marking the end time)
• closing the span and sending for processing (at which time it becomes immutable)

Design

BugsnagPerformanceSpan now has two new internal APIs:

• - (void)markEndTime:(NSDate *)endTime to set the end time of the span (can be called multiple times)
• - (void)sendForProcessing to send for processing (idempotent, only performs operation once)

Testing

Existing tests are sufficient since the public end functions now call these functions.

[github-actions]

BugsnagPerformance.framework binary size increased by 720 bytes from 524,168 to 524,888

    FILE SIZE        VM SIZE    
 --------------  -------------- 
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  +0.3%    +328  +0.3%    +328    String Table
  +0.2%    +204  +0.2%    +204    __TEXT,__text
  +0.8%    +144  +0.8%    +144    __DATA,__objc_const
  +0.7%    +115  +0.7%    +115    __TEXT,__objc_methname
  +0.8%     +96  +0.8%     +96    __TEXT,__objc_stubs
  +0.7%     +24  +0.7%     +24    __DATA,__objc_selrefs
  +0.1%     +12  +0.1%     +12    __TEXT,__gcc_except_tab
  +0.7%      +8  +0.7%      +8    Function Start Addresses
  +0.2%      +8  +0.2%      +8    __TEXT,__unwind_info
  +0.9%      +4  +0.9%      +4    __DATA,__objc_ivar
  -2.8%    -172  -2.9%    -172    [__DATA]
  -1.0%    -435  -1.0%    -435    [__TEXT]
  +0.1%    +720  +3.1% +16.0Ki    TOTAL

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