Add fastnum implementation

[GreeFine]

I added the implementation for Fastnum.

It's another crate for decimal numbers.
I could have done a more precise job on the De/Serialize regex for Unsigned values, but want to keep the changes to the minimum.

I did test it on my project, using aide, it matched expectations.

Workaround I used if it helps someone in the same situation

use std::borrow::Cow;

use schemars::{Schema, SchemaGenerator, generate::Contract};
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize, Clone, Copy)]
pub struct JsonUDecimal(fastnum::decimal::UnsignedDecimal<4>);

impl schemars::JsonSchema for JsonUDecimal {
    fn schema_name() -> Cow<'static, str> {
        "UDecimal".into()
    }

    fn json_schema(generator: &mut SchemaGenerator) -> Schema {
        let (ty, pattern) = match generator.contract() {
            Contract::Deserialize => ("string".into(), r"^\d+(\.\d+)?([eE]\d+)?$".into()),
            Contract::Serialize => ("string".into(), r"^\d+(\.\d+)?$".into()),
            // Contract is marked as non_exhaustive
            _ => unimplemented!("New version of Schemars introduced a new contract type"),
        };

        let mut result = Schema::default();
        result.insert("type".to_owned(), ty);
        result.insert("pattern".to_owned(), pattern);
        result
    }
}

impl From<fastnum::decimal::UnsignedDecimal<4>> for JsonUDecimal {
    fn from(value: fastnum::decimal::UnsignedDecimal<4>) -> Self {
        Self(value)
    }
}

[GreeFine]

Looks like Fastnum is requiring a Rust version that is quite new.
Sadly, dropping a few versions won't help, the 0.3 still requires Rust v1.87.0.