[feature] add 'Model::add_calendar' under 'mutable-model' feature

[woshilapin]

Introduce a new function to mutate the model.

Model::add_calendar(
  &mut self,
  calendar: Calendar,
  // Other parameters ?
)

[woshilapin]

This PR depends on hove-io/relational_types#11. However, there is still a bunch of discussions about that other PR and no consensus. Therefore, I'm not going to rely on this new OneToMany::add_link() function, but I'm going to recreate entirely the relation every time with OneToMany::new(). This might be less performant, but it will be always correct and performance improvement can still be addressed later if needed.