GetDevicePath get the path for a given device.

DeviceCertFilename the location in the device-specific directory that contains the device certificate.

DeviceOnboardFilename the location in the device-specific directory that contains the onboarding certificate used.

DeviceManager implementation of DeviceManager interface with a directory as the backing store.

DirReader reads the contents of all files in a directory, sorted by whatever the OS does and then lexicographically.