Msgpack and read/write functions don't handle the point at infnity for elements

[Rumata888]

Affine and group elements in barretenberg have a special point at infinity member which can't just be encoded as fields, which is what we currently do with msgpack.
Changing it requires updated to affine_element.hpp, serialize.hpp and same for the group element. However, one of the issues right now is that the read and write functions have to be reimplemented, but both affine and group elements can have different sizes, because they are implemented for extensions as well.