fix: prevent from adding duplicate public keys

[t-aleksander]

📖 Description

Prevents user from submitting a device public key that already exists in the database. Current behavior (lack of such constraint) can cause problems, as we assume in multiple places that all devices have unique keys. This new constraint is enforced only on the API level, as enforcing it on the database at this time could break existing end user setups

🛠️ Dev Branch Merge Checklist:

Documentation

• If testing requires changes in the environment or deployment, please update the documentation (https://defguard.gitbook.io) first and attach the link to the documentation section in this pool request
• I have commented on my code, particularly in hard-to-understand areas

Testing

• I have prepared end-to-end tests for all new functionalities
• I have performed end-to-end tests manually and they work
• New and existing unit tests pass locally with my changes

Deployment

• If deployment is affected I have made corresponding/required changes to deployment (Docker, Kubernetes, one-line install)

🏚️ Main Branch Merge Checklist:

Testing

• I have merged my changes before to dev and the dev checklist is done
• I have tested all functionalities on the dev instance and they work

Documentation

• I have made corresponding changes to the user & admin documentation and added new features documentation with screenshots for users/admins