feat: support loading device certificate from multiple locations (env, secrets, file)

[reubenmiller]

Support loading the device certificate from different sources:

• From base64 encoded environment variables (CERTPRIVATE and CERTPUBLIC)
• From docker secrets, called certificate_private_key and certificate_public_key
• From file (e.g. using a volume which contains an already created certificate)

The first found certificate (regardless if it is valid or not) will be taken.

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
12	0	1	12	100	1m18.020305s

Passed Tests

Name	⏱️ Duration	Suite
Grace period to allow container to startup	5.003 s	Operations
Get Logfile Request	2.348 s	Operations
Get Configuration File	4.644 s	Operations
Execute Shell Command	2.320 s	Operations
Install application using docker compose	21.527 s	Operations
Get Container Logs	2.324 s	Operations
Trigger self update via local command	11.415 s	Self-Update
Self update should only update if there is a new image	2.323 s	Self-Update
Self update using software update operation	23.325 s	Self-Update
Cloud Connection is Online	0.121 s	Telemetry
Service status	0.222 s	Telemetry
Sends measurements	2.447 s	Telemetry