code refactoring changes with macro attributes added to mapped existing fields

[douggynix]

This Pull request is the follow-up of this one with the pending required changes:
#11

The changes will add macro attributes as well for mapping struct fields:

 #[derive(DtoMapper, Debug, Default, Clone)]
#[mapper(
    dto="CustomDtoWithAttribute" ,
    no_builder=true ,
    map=[ ("email", false, ["#[serde(rename = \"email_address\")]"] ) ],
    derive=(Debug, Clone, Serialize, Deserialize),
    new_fields=[
        (
            "name: String",
            "concat_str( self.firstname.as_str(), self.lastname.as_str() )",
            ["#[serde(rename = \"full_name\")]"],
        ),
        (
          "hidden_password: String",
         r#""*".repeat( self.password.len() )"#
        ),
    ],
    macro_attr=["serde(rename_all = \"UPPERCASE\")"]
)]
    struct User {
        username: String,
        password: String,
        email: String,
        firstname: String,
        middle_name: Option<String>,
        lastname: String,
        age: u8,
    }

The macro definition above will generate this new struct with the attributes added in the mapping field.
Run this command to view the result:

cargo install cargo-expand
cargo expand --test dto_example

#[serde(rename_all = "UPPERCASE")]
pub struct CustomDtoWithAttribute {
    #[serde(rename = "email_address")]
    pub email: Option<String>,
    #[serde(rename = "full_name")]
    pub name: String,
    pub hidden_password: String,
}

[douggynix]

@fatihaziz: Feel free to comment, I am following up with your work with macro attributes on mapping fields.