allow custom Models in derive macro

[danieleades]

users should be able to use a custom model when using the derive macro.

something like this-

use minnow::Encodeable;

#[derive(Debug, Encodeable)]
pub struct Coordinates {
    #[encode(float(min = -10_000.0, max = 10_000.0, precision = 1))]
    pub x: f64,
    #[encode(model = MyCustomModel)]
    pub y: f64,
    #[encode(float(min = -5_000.0, max = 0.0, precision = 0))]
    pub z: f64,
}