[PLAT-5298] Remove breadcrumbs from client metadata

[nickdowell]

Goal

These changes reduce the CPU and memory impact of leaving breadcrumbs by stopping the breadcrumbs array from being serialised into an in-memory JSON buffer every time a new one is added.

Changeset

BSSerializeDataCrashHandler which is invoked in the event of an unhandled error now reads in the breadcrumbs data from the file on disk. Previously, the breadcrumbs were included in bsg_g_bugsnag_data.stateJSON

This required a slight change to where the breadcrumbs reside within the KSCrashReport JSON structure, so the BSGParseBreadcrumbs function was updated to reflect this.

Testing

A unit test was added to measure the performance of leaving breadcrumbs. This should report any regressions against the baseline as a test failure.

End-to-end tests check the breadcrumbs payload and these have passed.

Manually verified that breadcrumbs are still reported to the dashboard.

[kattrali]

looks good 🎉

Even though this doesn't have a public interface change, I'd recommend adding a changelog entry for this as its intended to address reported performance concerns and can be referenced in the future as a part of the next release.