[mlir][ods] Add convertFromStorage field to parameters

This patch adds a `convertFromStorage` field to attribute or type parameters that can implement more complex logic for converting from the parameter's C++ storage type (e.g. `Optional<SmallVector<T>>`) to its C++ type (e.g. `Optional<ArrayRef<T>>`).

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D128293