Tower Service Trait implementation

[omarabid]

This PR adds a Service struct that implements Tower Service Trait. This makes it possible to use crates that rely on this standard. The PR was only tested with Octocrab.

There is a minor change, too. Fetch::Request uses &self which leaves a dangling borrow. The PR changes this so that Fetch::Request consumes the original Fetch object. This should be harmless since Fetch is not returned anyway.

Example usage:

let service = worker::Service::default();

let octocrabi = octocrab::OctocrabBuilder::new_empty()
                .with_service(service)
                .with_layer(&BaseUriLayer::new(http::Uri::from_static(
                    "https://api.github.com",
                )))
                .with_layer(&ExtraHeadersLayer::new(Arc::new(vec![(
                    USER_AGENT,
                    "octocrab".parse().unwrap(),
                )])))
                .with_layer(&ExtraHeadersLayer::new(Arc::new(vec![(
                    "Authorization".parse().unwrap(),
                    "Bearer xxxxxxxxxxxxxxxxxxxxx"
                        .parse()
                        .unwrap(),
                )])))
                .with_auth(AuthState::None);