Implement From<MaybeUndefined<T>> for Option<Option<T>>

[dbstratta]

Hey @sunli829 ! Thanks for all the work you put into this crate. It's truly awesome.

Description of the feature

I think it would be useful to easily convert from MaybeUndefined<T> to Option<Option<T>>,
where Undefined => None, Null => Some(None) and Value(a) => Some(Some(a)).

I'm using the Option<Option<T>> pattern to tell in my SQL queries if I should set the value to null or to not update it at all (custom code in SQLx), and Diesel uses this pattern by default too.

Code example

This is the main type of use case I'm imagining:

struct UserChangeset {
  pub name: Option<Option<String>>,
}

#[Object]
impl Mutation {
  async fn change_name(&self, ctx: &Context<'_>, name: MaybeUndefined<String>) -> Result<()> {
    // ...
  
    user.update(UserChangeset { name: name.into() }).await
  }
}

What do you think? (I can implement the feature if you accept it)