[MP_EXT] Extension encoder and decoder functions are not exportable

[rybakit]

Consider making the current MessagePack extension encoder and decoder functions exportable. I’m personally interested in the decimal extension, where I would like to register my own extension but still be able to reuse the encoding logic from the tarantool/decimal package. The easiest way is to simply make the decimalEncoder() and decimalDecoder() functions start with a capital letter. Another option is to make them adhere to the MarshalerUnmarshaler interface from vmihailenco/msgpack/v5 and rename them to MarshalMsgpack() and UnmarshalMsgpack(), respectively.