API to access to the device.id field

[reubenmiller]

Is your feature request related to a problem? Please describe.

The Cumulocity opcua-device-gateway needs to get access to the device's Cumulocity external ID, however it runs in a container and does not generally have access to the filesystem where thin-edge.io is running. Without access to the file system, it is difficult for it to get the device's external ID except via using the Cumulocity proxy which requires the device to be currently connected to Cumulocity.

Describe the solution you'd like

Extend the entity store api to return the external id (@id) field of the devices.

Describe alternatives you've considered

Additional context

[reubenmiller]

Actually the use-cases also requires getting access to configured c8y.http value as well (in some modes of the opcua-device-gateway). Currently it gains access by mounting the /etc/tedge directory inside the container, and installing the tedge cli.