hyper_util: How to add `hyper_util::client::pool::map::Map` to custom struct?

[hatoo]

Is your feature request related to a problem? Please describe.

I want use hyper_util::client::pool::map::Map as connection cache as http client.
Because of map::builder::Built<> is not exposed, we can't fill generics parameter of hyper_util::client::pool::map::Map so we can't add it custom struct.

Describe the solution you'd like

No idea

Additional context

I want create a struct of HTTP client like

pub struct Client {
    ...
    pub client_pool: hyper_util::client::pool::map::Map<????>,
}

But I can't. Is there any alternative?