[PLAT-8411] Add device.freeMemory to OOM and Thermal Kill events

[nickdowell]

Goal

Provide recent memory statistics to improve confidence in OOM errors.

Changeset

Adds an availableMemory field to BSGRunContext and updates this in the timer callback (twice per second) and in response to a memory pressure notification.

Only on iOS/tvOS 13 and later where os_proc_available_memory() is available.

ℹ️ Does not update BSGRUNCONTEXT_VERSION since no release yet contains BSGRunContext.

Performance

os_proc_available_memory() is much faster than calling host_statistics() or task_info()... ~150 vs ~700 nanoseconds on an iPhone SE (2020).

Testing

Amends E2E scenarios to verify presence of device.freeMemory

[github-actions]

	1 Warning
⚠️	This PR modifies BSGRunContext.h but does not change BSGRUNCONTEXT_VERSION

Bugsnag.framework binary size increased by 120 bytes from 846,008 to 846,128

    FILE SIZE        VM SIZE    
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  +0.1%    +148  +0.1%    +148    __TEXT,__text
  +0.1%     +80  +0.1%     +80    Symbol Table
  +0.0%     +40  +0.0%     +40    String Table
  [ = ]       0  +0.2%     +16    __DATA,__bss
  +0.1%      +4  +0.1%      +4    __TEXT,__unwind_info
  [ = ]       0  -0.1%     -16    [__DATA]
  [ = ]       0  -2.0%    -120    [__LINKEDIT]
  -0.6%    -152  -0.6%    -152    [__TEXT]
  +0.0%    +120  [ = ]       0    TOTAL

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[kstenerud]

Can this be checked for an integer as well?

[nickdowell]

device.freeMemory is now checked as being an integer on line 239