feat(derive): add custom field for struct

[saiintbrisson]

Hi! It is not infrequently that I find myself needing to compute a custom
field that is only present on serialization and is calculated off of another
one of the struct's fields. Usually, I'll either implement Serialize myself,
but this gets tiring if the struct is too large and is prone to errors, or create
another struct with the field I want just before serializing.

I hereby propose creating a new container attribute field that allows for
the creation of fields on the go at the moment of serialization:

#[derive(Serialize)]
#[serde(
    field(name = "b", getter = "Self::b_getter"),
    field(name = "c", with = "c_serialize_with")
)]
struct CustomFields {
    a: i32
}

impl CustomFields {
    fn b_getter(&self) -> i32 {
        self.a * 2
    }
}

fn c_serialize_with<S: Serializer>(v: &CustomFields, ser: S) -> Result<S::Ok, S::Error> {
    (v.a * 3).serialize(ser)
}

So a CustomFields { a: 10 } results in:

{
  "a": 10,
  "b": 20,
  "c": 30
}

I'm still not happy with the attributes' names, it may make more sense to
call it serialize_field or something along those lines?

[dtolnay]

Thanks for the PR.

This was previously discussed and rejected in #760.