Support extensions in utoipa::IntoParams attribute macros

[aybeshlikyan]

Hello! This is my PR addressing the feature I proposed in Issue #1473, adding support to specifying OpenAPI extensions on parameters using the #[into_params] and #[param] attribute macros for structs that derive utoipa::IntoParams.
The implementation follows the form proposed in the issue:

#[derive(Debug, Clone, utoipa::IntoParams)]
#[into_params(parameter_in = Query, extensions(("x-on-all-params" = json!(true))))]
pub struct ParamStruct {
    /// A param.
    pub param_1: i32,

    /// Another param.
    #[param(extensions(("x-on-one-param" = json!({ "key": "value" }))))]
    pub param_2: String,
}

I updated the macro documentation, added unit tests for the various flavors, and updated the example introduced with the feature in #1292, simple-x-extensions.

I couldn't find any special docs about contribution guidelines, please let me know if I'm missing anything!

Closes issue #1473