Support Hrana backend without any HTTP backend features enabled

[george-lim]

The current implementation of InnerClient in http module prevents the hrana_backend feature from being enabled without having at least one other HTTP backend feature enabled.

This can be solved if we add conditional compilation to the http module only if a HTTP backend feature is enabled.

#[derive(Clone, Debug)]
pub enum InnerClient {
    #[cfg(feature = "reqwest_backend")]
    Reqwest(crate::reqwest::HttpClient),
    #[cfg(feature = "workers_backend")]
    Workers(crate::workers::HttpClient),
    #[cfg(feature = "spin_backend")]
    Spin(crate::spin::HttpClient),
}