feat(multiplayer): add Steam transport foundation

Game/scripts/steam_transport.gd

@@ -0,0 +1,44 @@
+class_name SteamTransport
+extends NetTransport
+
+const SteamBootstrapScript = preload("res://scripts/steam_bootstrap.gd")
+
+# The SteamMultiplayerPeer extension is looked up dynamically so a stock ENet
+# build never references an unavailable native class while parsing scripts.
+const VIRTUAL_PORT := 0
+
+func transport_id() -> String:
+	return "steam"
+
+
+func is_available() -> bool:
+	return SteamBootstrapScript.is_runtime_available()
+
+
+func unavailable_reason() -> String:
+	return SteamBootstrapScript.unavailable_reason()
+
+
+func create_server(_port: int, _max_clients: int) -> Dictionary:
+	var boot: Dictionary = SteamBootstrapScript.initialize()
+	if int(boot.error) != OK:
+		return boot
+	var peer := ClassDB.instantiate("SteamMultiplayerPeer") as MultiplayerPeer
+	if peer == null:
+		return {"error": ERR_UNAVAILABLE, "reason": "SteamMultiplayerPeer could not be instantiated"}
+	var err := int(peer.call("create_host", VIRTUAL_PORT))
+	return {"error": err, "peer": peer if err == OK else null, "reason": error_string(err)}
+
+
+func create_client(address: String, _port: int) -> Dictionary:
+	var steam_id_text := address.strip_edges()
+	if not steam_id_text.is_valid_int() or int(steam_id_text) <= 0:
+		return {"error": ERR_INVALID_PARAMETER, "reason": "Steam transport requires the server's numeric Steam ID"}
+	var boot: Dictionary = SteamBootstrapScript.initialize()
+	if int(boot.error) != OK:
+		return boot
+	var peer := ClassDB.instantiate("SteamMultiplayerPeer") as MultiplayerPeer
+	if peer == null:
+		return {"error": ERR_UNAVAILABLE, "reason": "SteamMultiplayerPeer could not be instantiated"}
+	var err := int(peer.call("create_client", int(steam_id_text), VIRTUAL_PORT))
+	return {"error": err, "peer": peer if err == OK else null, "reason": error_string(err)}