otConvertDurationInSecondsToString() function

Converts an `uint32_t` duration (in seconds) to a human-readable string. Requires `OPENTHREAD_CONFIG_UPTIME_ENABLE` to be enabled. The string follows the format "::" for hours, minutes, seconds (if duration is shorter than one day) or "d.::" (if longer than a day). If the resulting string does not fit in @p aBuffer (within its @p aSize characters), the string will be truncated but the outputted string is always null-terminated. Is intended for use with `mAge` or `mConnectionTime` in `otNeighborInfo` or `otChildInfo` structures.