[PLAT-6973] Fix stack overflow in +[BugsnagThread allThreadsWithCurrentThreadBacktrace:]

[nickdowell]

Goal

Fix a crash due to a stack overflow when recording thread backtraces in an app that has lots (~500+) threads.

Exception Type:     EXC_BAD_ACCESS 
Exception Subtype:  KERN_PROTECTION_FAILURE

EXC_BAD_ACCESS: Attempted to dereference garbage pointer 0x171e083b8.

0  AppName    bsg_ksbt_backtraceThreadState (BSG_KSBacktrace.c:145:28)
1  AppName    backtrace_for_thread (BugsnagThread.m:47:38)
2  AppName    +[BugsnagThread allThreadsWithCurrentThreadBacktrace:] (BugsnagThread.m:235:13)
3  AppName    +[BugsnagThread allThreads:callStackReturnAddresses:] (BugsnagThread.m:207:16)

This crash is less likely to occur on the main thread because it has a larger default stack size than background threads.

Changeset

Memory for the backtraces is now allocated from the heap rather than the stack.

Testing

Reproduced the crash in a unit test, verified it now passes.

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

Bugsnag.framework binary size did not change - 1,134,024 bytes

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