box: move Lua MP_EXT decoder from tuple.c

Lua C module 'msgpack' supports registration of custom extension
decoders for MP_EXT values. That is needed to make 'msgpack' not
depending on any modules which use it.

So far the only box-related extension were tuples - struct tuple
cdata needed to be encoded as an array.

That is going to change in next commits, where struct error cdata
appears, also depending on box. So the decoder can't be located
in src/box/lua/tuple.c. It is moved to a more common place -
src/box/lua/init.c.

Needed for #4398