Use Optional Types for GraphQL Optional

[maaft]

With go 1.18 generics are now possible to use.

These can be used for example to implement the Optional concept:
https://github.com/frenchie4111/go-generic-optional

Although the current workaround with input_as_map works, we also have following issues:

• When the order of arguments change for a GraphQL input object, we must migrate our existing code
• When the variable-name order changes but the type order does not, we won't even notice this through our linter
• auto-completion is not very good for function arguments

When instead we go back to structs but use the Optional concept from above package, we could have the best of both worlds:

• no order dependencies
• don't send optional values as nil