Add ClientBuilder and support a configureable reqwest client

[alexpusch]

cloud-storage-rs uses a default Reqwest client as its internal http client. Reqwest offers a large set of configuration that are inaccessible to cloud-storage-rs users.

This PR adds a ClientBuilder struct which enables to customize reqwest client

Example usage:

        let builder = Client::builder();
        let custom_client = reqwest::Client::builder()
            .timeout(core::time::Duration::from_secs(10))
            .build()?;

        let client = builder.with_reqwest_client(custom_client).build();

There are many use cases for providing a custom client:

• configuring proxy servers
• configuring timeouts, keepalive settings
• configuring custom http2 optimizations, or opt out to http1