token-server: Add protos for new API for generating service account tokens.

tokenserver/api/oauth_token_grant.proto

+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+syntax = "proto3";
+
+package tokenserver;
+
+import "google/protobuf/timestamp.proto";
+
+
+// OAuthTokenGrantBody contains the internal guts of an oauth token grant.
+//
+// It gets serialized, signed and stuffed into OAuthTokenGrantEnvelope, which
+// then also gets serialized to get the final blob with the grant. This blob is
+// then base64-encoded and returned to the caller of MintOAuthTokenGrant.
+message OAuthTokenGrantBody {
+  // Identifier of this token as generated by the token server.
+  //
+  // Used for logging and tracking purposes.
+  //
+  // TODO(vadimsh): It may later be used for revocation purposes.
+  int64 token_id = 1;
+
+  // Service account identity the end user wants to act as.
+  //
+  // A string of the form "user:<email>".
+  string service_account = 2;
+
+  // Who requested this token and who can pass it to MintOAuthTokenViaGrant.
+  //
+  // A string of the form "user:<email>". On Swarming, this is Swarming's own
+  // service account name.
+  string wielder_identity = 3;
+
+  // An end user that wants to act as the service account (perhaps indirectly).
+  //
+  // A string of the form "user:<email>". On Swarming, this is an identity of
+  // a user that posted the task.
+  string end_user_identity = 4;
+
+  // When the token was generated (and when it becomes valid).
+  google.protobuf.Timestamp issued_at = 5;
+
+  // How long the token is considered valid (in seconds).
+  //
+  // It may become invalid sooner if the token server policy changes and the
+  // new policy doesn't allow this token.
+  int64 validity_duration = 6;
+}
+
+
+// OAuthTokenGrantEnvelope is what is actually being serialized and send to
+// the callers of MintOAuthTokenGrant (after being encoded using base64 standard
+// raw encoding).
+message OAuthTokenGrantEnvelope {
+  bytes token_body       = 1; // serialized OAuthTokenGrantBody
+  string key_id          = 2; // id of a token server private key used for signing
+  bytes pkcs1_sha256_sig = 3; // signature of 'token_body'
+}