This add the basic APIs to create VariadicityAttr and
VariadicityArrayAttr attributes from the C API. This is necessary for
C API users that want to create IRDL dialect declarations.
Being able to add custom dialects is one of the big missing pieces of
the C API. This change should make it achievable via IRDL. Hopefully
this should open custom dialect definition to non-C++ users of MLIR.