feat: add validate attribute to InstructMacro

[drbh]

This PR adds a validate attribute and method to the InstructMacro macro.

validate simplifies the ability to write custom functions that "validate" each field. This can currently be done manually with serde(deserialize_with=...) however require verbose syntax.

This new macro attribute's usage can be seen in test_validation_macro or the updated getting started main.rs file

with this pr

#[derive(InstructMacro, Debug, Serialize, Deserialize)]
// This represents a single user
struct UserInfo {
    // This represents the name of the user
    #[validate(custom = "validate_uppercase")]
    name: String,
    // This represents the age of the user
    age: u8,
}

#[validate]
fn validate_uppercase(s: &String) -> Result<String, String> {
    if s.chars().any(|c| c.is_lowercase()) {
        return Err(format!(
            "Name '{}' should be entirely in uppercase. Examples: 'TIMOTHY', 'JANE SMITH'",
            s
        ));
    }
    Ok(s.to_uppercase())
}

// ...

println!("{}", result.name); // JOHN DOE
println!("{}", result.age); // 30

instead of

#[derive(InstructMacro, Debug, Serialize, Deserialize)]
// This represents a single user
struct UserInfo {
    // This represents the name of the user
    #[serde(deserialize_with = "validate_uppercase")]
    name: String,
    // This represents the age of the user
    age: u8,
}

fn validate_uppercase<'de, D>(de: D) -> Result<String, D::Error>
where
    D: Deserializer<'de>,
{
    let s = String::deserialize(de)?;
    if s.chars().any(|c| c.is_lowercase()) {
        return Err(de::Error::custom(format!(
            "Name '{}' should be entirely in uppercase. Examples: 'TIMOTHY', 'JANE SMITH'",
            s
        )));
    }
    Ok(s.to_uppercase())
}

// ...

println!("{}", result.name); // JOHN DOE
println!("{}", result.age); // 30

**note this PR includes a couple small refactors and changes the dependencies to relative paths (this is only done to ease the ability to test this PR). Please let me know if any changes should be made or if this functionality is not aligned with this library.

Thank you!!

[ivanleomk]

Pushed up some changes, lgtm! Thanks for contributing