agessh: support PKCS#8-encoded Ed25519 private keys

OpenSSH never generated them (unencrypted, and golang.org/x/crypto/ssh
doesn't support encrypted PKCS#8 for now, so the encrypted_keys.go
change is technically superfluous) but there are other systems that
produce them (for example, 1Password). Unfortunately, ParseRawPrivateKey
returns a value type for PKCS#8 and a pointer type for the OpenSSH
format (golang/go#51974), so we need to handle both.

Fixes #429

cmd/age/testdata/pkcs8.txt

@@ -0,0 +1,12 @@
+# https://github.com/FiloSottile/age/discussions/428
+# encrypt and decrypt a file with an Ed25519 key encoded with PKCS#8
+age -e -i key.pem -o test.age input
+age -d -i key.pem test.age
+stdout test
+
+-- input --
+test
+-- key.pem --
+-----BEGIN PRIVATE KEY-----
+MC4CAQAwBQYDK2VwBCIEIJT4Wpo+YG11yybKL/bYXQW7ekz4PAsmV/4tfmY1vU7x
+-----END PRIVATE KEY-----