Add gRPC Java S2A protos (#149)

Context:
- grpc/grpc-java#11113
- https://github.com/google/s2a-go/blob/main/README.md

grpc/gcp/s2a/common.proto

@@ -0,0 +1,82 @@
+// Copyright 2024 The gRPC Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// The canonical version of this proto can be found at
+// https://github.com/grpc/grpc-proto/blob/master/grpc/gcp/s2a/common.proto
+
+syntax = "proto3";
+
+package grpc.gcp.s2a;
+
+option java_multiple_files = true;
+option java_outer_classname = "CommonProto";
+option java_package = "io.grpc.s2a.handshaker";
+
+// The TLS 1.0-1.2 ciphersuites that the application can negotiate when using
+// S2A.
+enum Ciphersuite {
+  CIPHERSUITE_UNSPECIFIED = 0;
+  CIPHERSUITE_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = 1;
+  CIPHERSUITE_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = 2;
+  CIPHERSUITE_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 = 3;
+  CIPHERSUITE_ECDHE_RSA_WITH_AES_128_GCM_SHA256 = 4;
+  CIPHERSUITE_ECDHE_RSA_WITH_AES_256_GCM_SHA384 = 5;
+  CIPHERSUITE_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256 = 6;
+}
+
+// The TLS versions supported by S2A's handshaker module.
+enum TLSVersion {
+  TLS_VERSION_UNSPECIFIED = 0;
+  TLS_VERSION_1_0 = 1;
+  TLS_VERSION_1_1 = 2;
+  TLS_VERSION_1_2 = 3;
+  TLS_VERSION_1_3 = 4;
+}
+
+// The side in the TLS connection.
+enum ConnectionSide {
+  CONNECTION_SIDE_UNSPECIFIED = 0;
+  CONNECTION_SIDE_CLIENT = 1;
+  CONNECTION_SIDE_SERVER = 2;
+}
+
+// The ALPN protocols that the application can negotiate during a TLS handshake.
+enum AlpnProtocol {
+  ALPN_PROTOCOL_UNSPECIFIED = 0;
+  ALPN_PROTOCOL_GRPC = 1;
+  ALPN_PROTOCOL_HTTP2 = 2;
+  ALPN_PROTOCOL_HTTP1_1 = 3;
+}
+
+message Identity {
+  oneof identity_oneof {
+    // The SPIFFE ID of a connection endpoint.
+    string spiffe_id = 1;
+
+    // The hostname of a connection endpoint.
+    string hostname = 2;
+
+    // The UID of a connection endpoint.
+    string uid = 4;
+
+    // The username of a connection endpoint.
+    string username = 5;
+
+    // The GCP ID of a connection endpoint.
+    string gcp_id = 6;
+  }
+
+  // Additional identity-specific attributes.
+  map<string, string> attributes = 3;
+}