Feat/add generation of device tls cert

[giusebar]

Devices registering to COS for devices might require the generation of certificates to be used in their apps (e.g. a TCP server running on the robot or a websocket).
This PR adds the generation of self-signed TLS certificates when a GET request is received on the api/v1/devices/<uuid>/certificate endpoint.
The cert and key are then sent in the response.

Tests and openapi have been updated accordingly.

An example of the agent registering and retrieving certificates is available in canonical/cos-registration-agent#53.

[Guillaumebeuzeboc]

I am not sure about the logic. It appears to be opposed of what was done before.
Previously, the cos-registration-agent was creating its UUID if not provided and simply registering it to the server.
Same for the public/private RSA key we were using for the ros2bag-filserver.
Here the logic is the opposite.
We have been using the POST api/v1/devices to store information to the server. And here we are requesting an information from the server.
Maybe we could add an additional endpoint to request a certificate generation (something like GET api/v1/devices/UUID/certificate). This way we don't have one endpoint that does too many things.

[giusebar]

I am not sure about the logic. It appears to be opposed of what was done before. Previously, the cos-registration-agent was creating its UUID if not provided and simply registering it to the server. Same for the public/private RSA key we were using for the ros2bag-filserver. Here the logic is the opposite. We have been using the POST api/v1/devices to store information to the server. And here we are requesting an information from the server. Maybe we could add an additional endpoint to request a certificate generation (something like GET api/v1/devices/UUID/certificate). This way we don't have one endpoint that does too many things.

This has been addressed and changed after discussion, from commit 536e4c4