Add object encoding param to serialization methods

Network peers get_var/put_var
File get_var/store_var
GDScript/Mono/VisualScript bytes2var/var2bytes
Add MultiplayerAPI.allow_object_decoding member which deprecates PacketPeer.allow_object_decoding.

Break ABI compatibaility (API compatibility for GDNative).

(cherry picked from commit 393e62b9)