feat(client): add default header middleware

[joelwurtz]

This PR add a new middleware to set default headers for client, avoiding repeating the same calls when we are sure that some headers should be always there

[fakeshadow]

Thanks for the PR.

In general we want to avoid adding too much default middleware unless it has complicated internal logic that end user may have difficulty implementing themselves. The default header one seems not belonging into that category.

I do get applying headers to the request API is not a good experience and I guess my opinion is we should consider making writing it easier without adding a dedicated middleware.

What do you think if we add middleware_fn API where an async closure is accepted as middleware like how xitca-web hehave. And adding default headers will become something like

Client::builder()
    .middleware_fn(async |req, srv| {
        req.headers_mut().insert();
        srv.call(req).await
    })

This would offer the same functionality as the middleware proposed by this PR and on top of it enables closure capture for stateful headers constructing on fly.

[joelwurtz]

I think that's a good idea, I also want to add a better api on the request builder to add header with a fluent interface (returning Self)

[joelwurtz]

Should the client reuse the xitca service crate ? or should we duplicate this logic ? i think reusing all the service / middleware logic from xitca-service would help a lot here

[fakeshadow]

I explained it in previous discussion #1164 (comment)

Due to current Rust's limitation there is no way to unify the Service trait between thread safe and non thread safe use case. Rust features like RTN and/or TAIT are required to reuse a single abstraction. For now duplicating the trait is the only solution that work on stable Rust.

[joelwurtz]

Closed in favor of #1182