Support extensions in utoipa::ToSchema named structs

[TilBlechschmidt]

Hi! 👋

With this PR I would like to add the support for OpenAPI extensions to the ToSchema macro. The functionality is limited to named structs as I was unsure about the implications of enabling it for the other kinds as well.

#[derive(ToSchema)]
struct HelloWorld {
    #[schema(extensions(
        ("x-terraform-requires-replace" = json!(true))
    ))]
    id: u32
}

Due to the prior work, the change is rather minimal and simply ties in the existing attribute parser into the schema macro.

---

Prior work:

• PR Issue #1228 - Feature: support extensions in utoipa::path macro #1292 added support for OpenAPI extensions. This is currently limited to the path macro.
• PR Support extensions in utoipa::IntoParams attribute macros #1474 extends this with support for the IntoParams macro.