[spinel] add support for logging crash dumps (#10061)

This commit adds a new feature that allows platforms to log crash logs.

### Additions
- `void otPlatLogCrashDump(void)` - API that logs a crash dump using
  OpenThread Logging APIs
- `SPINEL_PROP_RCP_LOG_CRASH_DUMP` - spinel prop that calls
  `otPlatLogCrashDump()` when `Set`
- `SPINEL_CAP_RCP_LOG_CRASH_DUMP` - spinel capability denoting that a
  RCP supports logging crash dumps

### Usage
- For `ot-cli-ftd|mtd`, `otPlatLogCrashDump()` is called at the end of
  app initialization, before the main loop. See [main.c]
- For Host/RCP setups, during initialization, the Host gets the RCP
  capabilities. If the RCP has the `SPINEL_CAP_RCP_LOG_CRASH_DUMP`
  capability, the Host will `Set` the `SPINEL_PROP_RCP_LOG_CRASH_DUMP`
  property, triggering the RCP to call `otPlatLogCrashDump()`.
  - If RCP Recovery is enabled, the this will also happen once the RCP
    has been restored

tests/unit/test_platform.cpp

@@ -844,4 +844,8 @@ OT_TOOL_WEAK void otPlatDnssdUnregisterKey(otInstance                 *aInstance
 
 #endif // OPENTHREAD_CONFIG_PLATFORM_DNSSD_ENABLE
 
+#if OPENTHREAD_CONFIG_PLATFORM_LOG_CRASH_DUMP_ENABLE
+OT_TOOL_WEAK otError otPlatLogCrashDump(void) { return OT_ERROR_NONE; }
+#endif
+
 } // extern "C"