Add ability to encrypt CA private key at rest (#386)

Fixes #8.

`nebula-cert ca` now supports encrypting the CA's private key with a
passphrase. Pass `-encrypt` in order to be prompted for a passphrase.
Encryption is performed using AES-256-GCM and Argon2id for KDF. KDF
parameters default to RFC recommendations, but can be overridden via CLI
flags `-argon-memory`, `-argon-parallelism`, and `-argon-iterations`.

cert/cert.proto

@@ -26,4 +26,22 @@ message RawNebulaCertificateDetails {
 
     // sha-256 of the issuer certificate, if this field is blank the cert is self-signed
     bytes Issuer = 9;
-}
+}
+
+message RawNebulaEncryptedData {
+	RawNebulaEncryptionMetadata EncryptionMetadata = 1;
+	bytes Ciphertext = 2;
+}
+
+message RawNebulaEncryptionMetadata {
+	string EncryptionAlgorithm = 1;
+	RawNebulaArgon2Parameters Argon2Parameters = 2;
+}
+
+message RawNebulaArgon2Parameters {
+	int32 version = 1; // rune in Go
+	uint32 memory = 2;
+	uint32 parallelism = 4; // uint8 in Go
+	uint32 iterations = 3;
+	bytes salt = 5;
+}