auth-api: move to Ed25519 signatures

auth-api: move to Ed25519 signatures

previously we used P-256 as the curve of our choice for ec signatures.
however, in the meantime Ed25519 has become a lot more wide-spread.
this simplifies our ec generation code significantly while keeping the
same security level. Ed25519 was also specifically designed and
reviewed to avoid implementation errors likely making it a more secure
choice

note that Ed25519 as a signature scheme always uses sha512, so signing
or verifying with a chosen digest is not supported.

as this mostly affects newly generated keys, this should not break any
existing setups.

Signed-off-by: Stefan Sterz <s.sterz@proxmox.com>