swanctl: Support any key type for decrypted keys

swanctl: Support any key type for decrypted keys

The previous code required explicit support for a particular key type,
of which Ed25519 and Ed448 were missing.  While a fallback to `any` would
have been possible (this is already the case for unencrypted keys in the
`private` and `pkcs8` directories, which are not parsed by swanctl), it's
not necessary (as long as swanctl and the daemon are from the same release)
and does not require the daemon to detect the key type again.

Fixes #3586.