Adds injectable HTTP requester support to client

[jobezic]

Introduces a trait for HTTP requesters, allowing custom implementations to be injected into the client.
Enables greater flexibility and testability by decoupling HTTP logic from the client core.

[ethanndickson]

Thanks for the PR! I hope this library can be of some use :)

The Rust ecosystem has changed a lot since this was written, and core parts of the library like our HTTP Client code could do with some modernizing & generally some love. 🥲

[ethanndickson]

Instead of replacing the enum here, could we perhaps extend it?
IIRC, something like this could work:

pub(crate) enum ClientInner {
    Https(HTTPSClient),
    Http(HTTPClient),
    Custom(Arc<dyn HttpRequester>),
}

[jobezic]

sounds like a good idea, check my last changes please.