Consider changing interface of internal send_request function

[thomaseizinger]

One of the design goals of this client is to allow for easy generation of proxies by implementing the trait yourself and forwarding to an inner implementation.

At the moment, this doesn't work quite well because the generated send_request function doesn't do the parsing of the response and hence anyone overwriting that function cannot access stuff like the JsonRpcError.

[gggto]

Regarding this issue, shouldn't the SendRequest trait look something along these lines:

#[async_trait::async_trait]
pub trait SendRequest: 'static
{
    type Error: StdError;

    async fn send_request<P>(
        &self,
        endpoint: Url,
        body: String,
    ) -> Result<Response<P>, Error<Self::Error>>
    where
        P: DeserializeOwned;
}

Where the From requirement is gone, the resulting Err being a jsonrpc_client::Error. That coupled with a general impl for Error casting ergonomics:

impl<T> From<T> for crate::Error<T> {
    fn from(inner: T) -> Self {
        crate::Error::Client(inner)
    }
}

With this, we would:

1. Be able to Implement SendRequest with errors outside the crate,
2. Still be able to cast the inner error type to the jsonrpc_client error type,
3. Be able to report parsing errors of the response json with the JsonRpc error variant.

Or, another take would be that maybe parsing the json response should be handled by the caller, replacing the Response<P> in the return type with maybe a String or something. In that case, we would do something like that:

#[async_trait::async_trait]
pub trait SendRequest: 'static
{
    type Error: StdError;

    async fn send_request(
        &self,
        endpoint: Url,
        body: String,
    ) -> Result<String, Self::Error>
}

While keeping the impl<T> From<T> for crate::Error<T>. But that second option is more subject to debate IMHO, not sure what is the right thing to do.

[thomaseizinger]

Feel free to give the first one a shot, I don't remember whether I just didn't try this or if there are any issues why it doesn't work!

Happy to discuss over a draft PR as well 😊