[PLAT-8410] Add device.time to OOM and Thermal Kill events

[nickdowell]

Goal

Provide a timestamp for OOM and Thermal Kill events so that the dashboard renders breadcrumbs with sensible relative timestamps.

Changeset

Adds a timestamp field to BSGRunContext which is updated from a timer and in response to certain events (so that device.time >= breadcrumbs.last.timestamp)

Sets device.time for OOM and TK events based on the last run's context.

Uses CFAbsoluteTimeGetCurrent() as the source of timing information, which provides a wall-clock time equivalent to [NSDate date].timeIntervalSinceReferenceDate.

Performance

CFAbsoluteTime() is very fast on recent OS versions (~70 nanoseconds on iPhone 5s with iOS 12) where it does not need to call into the kernel.

On iPad 3 running iOS 9 it takes ~1400 nanoseconds (1.4 µs) which while much slower is still reasonable for the frequency at which this code calls it. For comparison, a single character write() to stdout takes ~8 µs.

Considered using mach_approximate_time() which is much faster (~45 nanoseconds on iPad 3) but since it would be difficult (perhaps impossible?) to convert to the required wall-clock time and only old devices / OSes would benefit, the extra complexity does not feel justified.

Testing

Amends OOM barebone E2E scenario to verify presence of device.time.

Manually verified outcome on dashboard using example app.

[github-actions]

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

Bugsnag.framework binary size increased by 648 bytes from 845,360 to 846,008

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[nickdowell]

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

Have not changed BSGRUNCONTEXT_VERSION because we have not yet made a release that includes BSGRunContext