feature: Middleware Architecture

[gakonst]

Introduces a new Middleware trait which is implemented by Provider. Anyone that writes a new middleware needs to implement it, like below:

struct MyStruct<M>(M);

#[derive(Error, Debug)]
pub enum MyError<M: Middleware> {
    #[error("{0}")]
    MiddlewareError(M::Error),

    // Add your middleware's specific errors here
}

#[async_trait(?Send)]
impl<M> Middleware for MyStruct<M>
where
    M: Middleware,
{
    type Error = MyError<M>;
    type Provider = M::Provider;
    type Inner = M;

    fn inner(&self) -> &M {
        &self.0
    }
 
   // Add any of the functions you want to override e.g. a logging middleware
}