Add and use ed25519 helper function from ostree test

Copy ed25519 helper function from ostree test to update-manager-common.sh,
this will allow to share it between all test files.
Generateed25519 keys to sign and verify static delta superblock during
the test. Save the public key to its own file.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>