*: Support Neo N3 auth scheme

This adds new `refs.SignatureScheme` enum value making any
`refs.Signature` field work as N3 witness. This feature will allow
authentication and authorization with multi-signatures and
contract-based accounts.

Closes #305.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

proto-docs/refs.md

@@ -149,8 +149,8 @@ Signature of something in NeoFS.
 
 | Field | Type | Label | Description |
 | ----- | ---- | ----- | ----------- |
-| key | [bytes](#bytes) |  | Public key used for signing |
-| sign | [bytes](#bytes) |  | Signature |
+| key | [bytes](#bytes) |  | Public key used for signing. For N3 `scheme`, the field represents a verification script. |
+| sign | [bytes](#bytes) |  | Signature. For N3 `scheme`, the field represents an invocation script. |
 | scheme | [SignatureScheme](#neo.fs.v2.refs.SignatureScheme) |  | Scheme contains digital signature scheme identifier |
 
 
@@ -223,6 +223,7 @@ Signature scheme describes digital signing scheme used for (key, signature) pair
 | ECDSA_SHA512 | 0 | ECDSA with SHA-512 hashing (FIPS 186-3) |
 | ECDSA_RFC6979_SHA256 | 1 | Deterministic ECDSA with SHA-256 hashing (RFC 6979) |
 | ECDSA_RFC6979_SHA256_WALLET_CONNECT | 2 | Deterministic ECDSA with SHA-256 hashing using WalletConnect API. Here the algorithm is the same, but the message format differs. |
+| N3 | 3 | Neo N3 witness. |
 
 
  <!-- end enums -->

refs/types.proto

@@ -105,9 +105,10 @@ message Version {
 
 // Signature of something in NeoFS.
 message Signature {
-  // Public key used for signing
+  // Public key used for signing. For N3 `scheme`, the field represents a
+  // verification script.
   bytes key = 1 [json_name = "key"];
-  // Signature
+  // Signature. For N3 `scheme`, the field represents an invocation script.
   bytes sign = 2 [json_name = "signature"];
   // Scheme contains digital signature scheme identifier
   SignatureScheme scheme = 3 [json_name = "scheme"];
@@ -124,6 +125,9 @@ enum SignatureScheme {
   // Deterministic ECDSA with SHA-256 hashing using WalletConnect API.
   // Here the algorithm is the same, but the message format differs.
   ECDSA_RFC6979_SHA256_WALLET_CONNECT = 2;
+
+  // Neo N3 witness.
+  N3 = 3;
 }
 
 // RFC 6979 signature.