Add proc-macro for paths

[LazyMechanic]

Instead of describing the paths in the describe_api macro, I would like to have some proc-macro.

Instead of this:

describe_api! {
    // ...
    paths: {
        ("some" / "path" / { param: String }): {
            GET: {
                description: "Get smth",
                200: Smth,
                500: ApiError
            },
            POST: {
                description: "Do smth",
                200: None,
                500: ApiError
            },
        }
    }
}

describe api like this:

#[opg::path(
    path = "some/path/{param}",
    method = "GET",
    description = "Get smth",
    on_200 = "Smth",
    on_500 = "ApiError",
)]
async fn get_smth() -> Result<Smth, ApiError> {...}

#[opg::path(
    path = "some/path/{param}",
    method = "POST",
    description = "Get smth",
    on_200 = "None",
    on_500 = "ApiError",
)]
async fn do_smth() -> Result<(), ApiError> {...}

// ...

describe_api! {
    // ...
    paths: {
        get_smth,
        do_smth,
    }
}