Don't rely on accessing the public device certificate to get the device's common name

[reubenmiller]

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

In instances where mosquitto is not being run on the same device as the thin-edge components (e.g. in the containerization scenario), the thin-edge.io components currently rely on access to the public device's certificate to get the external identity (required for some API calls to Cumulocity).

Describe the solution you'd like

The thin-edge.io components should be able to check what the device's common name via MQTT or REST. It should not rely on having access to the device's public certificate.

Describe alternatives you've considered

Additional context

[reubenmiller]

This has been solved for the following components:

• [x] tedge-agent
• [x] tedge-log-plugin
• [x] tedge-configuration-plugin

Remaining:

• [ ] c8y-firmware-plugin (or the after the firmware plugin is made generic and the plugin can be deprecated)

Not Applicable:

• tedge-mapper (c8y) - The mapper needs to be able to read the certificate as when the Cumulocity REST API support cert based authentication, then it will need access to the certificates as well