luci-mod-system: allow ecdsa-sk and ed25519-sk key types

Allow adding ecdsa-sk and ed25519-sk SSH keys in LuCI
These key types can be generated via the -t flag in ssh-keygen and are
supported in recent versions of dropbear. As ssh-keygen ignores the -b
flag when generating ecdsa-sk and ed25519-sk keys, the curve field in
the objects returned by the decode function is set to fixed strings for
both ecdsa-sk and ed25519-sk public key strings. This is in contrast to
ecdsa keys for which various curves can be provided (e.g., NIST P-256,
NIST P-384, and NIST P-521).

Signed-off-by: Eric McDonald <ericmcdonald@protonmail.com>