Make Option non-required & add required attr

[juhaku]

Make Option fields non-required by default. This is the way it used to be originally but was changed due hopes for better client code generation. However since it caused issues with client side generation the behaviour is now changed back on this commit.

The nullability behaviour will still stay to same, so Option will still be considered nullable. These rules are explained in the docs for greater detail.

This commit also adds required attribute that can be used to declare a field of ToSchema and IntoParams as required in order to change the default required status of a field / parameter.

Since Option is by default non-required, this would enforce the name parameter to be required.

 #[derive(IntoParams)]
 #[into_params(parameter_in = Query)]
 struct Params {
     #[param(required)]
     name: Option<String>,
 }

Fixes #529