Add a preliminary gRPC service for dealing with calling credentials

service/src/main/proto/org/signal/chat/calling.proto

@@ -0,0 +1,41 @@
+syntax = "proto3";
+
+option java_multiple_files = true;
+
+package org.signal.chat.calling;
+
+/**
+ * Provides methods for getting credentials for one-on-one and group calls.
+ */
+service Calling {
+
+  /**
+   * Generates and returns TURN credentials for the caller.
+   *
+   * This RPC may fail with a `RESOURCE_EXHAUSTED` status if a rate limit for
+   * generating TURN credentials has been exceeded, in which case a
+   * `retry-after` header containing an ISO 8601 duration string will be present
+   * in the response trailers.
+   */
+  rpc GetTurnCredentials(GetTurnCredentialsRequest) returns (GetTurnCredentialsResponse) {}
+}
+
+message GetTurnCredentialsRequest {}
+
+message GetTurnCredentialsResponse {
+  /**
+   * A username that can be presented to authenticate with a TURN server.
+   */
+  string username = 1;
+
+  /**
+   * A password that can be presented to authenticate with a TURN server.
+   */
+  string password = 2;
+
+  /**
+   * A list of TURN (or TURNS or STUN) servers where the provided credentials
+   * may be used.
+   */
+  repeated string urls = 3;
+}