session: change issuer of token v2 to OwnerID

Since the issuer cannot be indirect (from NNS) because the token must be signed
by the user, the issuer must be a `OwnerID` type.

Signed-off-by: Andrey Butusov <andrey@nspcc.io>

Author: End-rey

proto-docs/session.md

@@ -317,7 +317,7 @@ Session Token body.
 | ----- | ---- | ----- | ----------- |
 | version | [uint32](#uint32) |  | Token version. |
 | nonce | [uint32](#uint32) |  | Random nonce to prevent collisions. |
-| issuer | [Target](#neo.fs.v2.session.Target) |  | Account that issued this token (who signed it). |
+| issuer | [neo.fs.v2.refs.OwnerID](#neo.fs.v2.refs.OwnerID) |  | Account that issued this token (who signed it). |
 | subjects | [Target](#neo.fs.v2.session.Target) | repeated | Accounts authorized by this token (who can use it). Maximum number of subjects: 8. |
 | lifetime | [TokenLifetime](#neo.fs.v2.session.TokenLifetime) |  | Lifetime of this token. |
 | contexts | [SessionContextV2](#neo.fs.v2.session.SessionContextV2) | repeated | Unified session contexts for both object and container operations. Multiple contexts allow authorization for different combinations. Maximum number of contexts: 16. |

session/types.proto

@@ -352,7 +352,7 @@ message SessionTokenV2 {
     uint32 nonce = 2 [json_name = "nonce"];
 
     // Account that issued this token (who signed it).
-    Target issuer = 3 [json_name = "issuer"];
+    neo.fs.v2.refs.OwnerID issuer = 3 [json_name = "issuer"];
 
     // Accounts authorized by this token (who can use it).
     // Maximum number of subjects: 8.