Add the test for ed25519 signed update
Check the public key availability and remote configuration.
Upgrade cases:
- trying to upgrade with static bundle without signed commit
- trying to upgrade with static bundle signed with unknown key
- check the correct upgrade bundle
Test run on x86_64 HW (all checks passed but can't boot properly after upgrade -- known issue for x86_64): https://lava.collabora.co.uk/results/1830593