proto: add AuthRegister and AuthApprove RPCs

Add gRPC service definitions for managing auth requests:
AuthRegister to register interactive auth sessions and
AuthApprove/AuthReject to approve or deny pending requests
(used for SSH check mode).

Updates #1850

proto/headscale/v1/auth.proto

@@ -0,0 +1,26 @@
+syntax = "proto3";
+package headscale.v1;
+option go_package = "github.com/juanfont/headscale/gen/go/v1";
+
+import "headscale/v1/node.proto";
+
+message AuthRegisterRequest {
+  string user = 1;
+  string auth_id = 2;
+}
+
+message AuthRegisterResponse {
+  Node node = 1;
+}
+
+message AuthApproveRequest {
+  string auth_id = 1;
+}
+
+message AuthApproveResponse {}
+
+message AuthRejectRequest {
+  string auth_id = 1;
+}
+
+message AuthRejectResponse {}