[api] add `otPlatLogCrashDump()` #1384

Job	Run time
Unittests	3m 1s
Examples	2m 30s
Stress network-forming	21m 58s
Stress commissioning	17m 1s
Stress connectivity	17m 24s
Stress network-latency	2m 37s
Stress multicast-performance	2m 17s
Stress otns-performance	2m 21s
upload-coverage	14s
	1h 9m 23s

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