Split behavior for request deserialization and response serialization

[Rexagon]

Add overrides for top level attribute meta list:

#[derive(Serialize, Deserialize, OpgModel)]
pub struct Test {
    #[serde(serialize_with = "produces_string", deserialize_with = "produces_int")]
    #[opg(res("Just response"), req("Just request", integer))]
    field: String,
}