[Net] Modularize multiplayer, expose MultiplayerAPI to extensions.

- RPC configurations are now dictionaries.
- Script.get_rpc_methods renamed to Script.get_rpc_config.
- Node.rpc[_id] and Callable.rpc now return an Error.
- Refactor MultiplayerAPI to allow extension.
- New MultiplayerAPI.rpc method with Array argument (for scripts).
- Move the default MultiplayerAPI implementation to a module.

core/SCsub

@@ -188,7 +188,6 @@ SConscript("os/SCsub")
 SConscript("math/SCsub")
 SConscript("crypto/SCsub")
 SConscript("io/SCsub")
-SConscript("multiplayer/SCsub")
 SConscript("debugger/SCsub")
 SConscript("input/SCsub")
 SConscript("variant/SCsub")

core/core_constants.cpp

@@ -31,7 +31,6 @@
 #include "core_constants.h"
 
 #include "core/input/input_event.h"
-#include "core/multiplayer/multiplayer.h"
 #include "core/object/class_db.h"
 #include "core/os/keyboard.h"
 #include "core/variant/variant.h"
@@ -660,15 +659,6 @@ void register_global_constants() {
 	BIND_CORE_ENUM_CONSTANT(METHOD_FLAG_OBJECT_CORE);
 	BIND_CORE_ENUM_CONSTANT(METHOD_FLAGS_DEFAULT);
 
-	// rpc
-	BIND_CORE_ENUM_CONSTANT_CUSTOM("RPC_MODE_DISABLED", Multiplayer::RPC_MODE_DISABLED);
-	BIND_CORE_ENUM_CONSTANT_CUSTOM("RPC_MODE_ANY_PEER", Multiplayer::RPC_MODE_ANY_PEER);
-	BIND_CORE_ENUM_CONSTANT_CUSTOM("RPC_MODE_AUTHORITY", Multiplayer::RPC_MODE_AUTHORITY);
-
-	BIND_CORE_ENUM_CONSTANT_CUSTOM("TRANSFER_MODE_UNRELIABLE", Multiplayer::TRANSFER_MODE_UNRELIABLE);
-	BIND_CORE_ENUM_CONSTANT_CUSTOM("TRANSFER_MODE_UNRELIABLE_ORDERED", Multiplayer::TRANSFER_MODE_UNRELIABLE_ORDERED);
-	BIND_CORE_ENUM_CONSTANT_CUSTOM("TRANSFER_MODE_RELIABLE", Multiplayer::TRANSFER_MODE_RELIABLE);
-
 	BIND_CORE_ENUM_CONSTANT_CUSTOM("TYPE_NIL", Variant::NIL);
 	BIND_CORE_ENUM_CONSTANT_CUSTOM("TYPE_BOOL", Variant::BOOL);
 	BIND_CORE_ENUM_CONSTANT_CUSTOM("TYPE_INT", Variant::INT);