chainHead based backend implementation (#1161)

* add follow_stream impl

* follow_stream_unpin first draft

* add tests for follow_stream_unpin

* more tests and fixes for follow_stream_unpin

* first pass follow_stream_driver

* follow_stream_driver: add tests, fix things, buffer events from last finalized

* First pass finishing Backend impl

* Fix test compile issues

* clippy fixes

* clippy fix and consistify light_client

* revert lightclient tweak

* revert other lightclient thing

* cargo fmt

* start testing unstable backend behind feature flag

* more test fixes and move test-runtime metadata path just incase

* fix compile error

* ensure transaction progress stream actually used and fix another test

* cargo fmt

* CI tweak

* improve some comments and address some feedback bits

* update CI to use our own nightly binary

* wait for finalized block perhaps

subxt/src/error/mod.rs

@@ -108,11 +108,22 @@ pub enum RpcError {
     /// Error related to the RPC client.
     #[error("RPC error: {0}")]
     ClientError(Box<dyn std::error::Error + Send + Sync + 'static>),
+    /// This error signals that the request was rejected for some reason.
+    /// The specific reason is provided.
+    #[error("RPC error: request rejected")]
+    RequestRejected(String),
     /// The RPC subscription dropped.
     #[error("RPC error: subscription dropped.")]
     SubscriptionDropped,
 }
 
+impl RpcError {
+    /// Create a `RequestRejected` error from anything that can be turned into a string.
+    pub fn request_rejected<S: Into<String>>(s: S) -> RpcError {
+        RpcError::RequestRejected(s.into())
+    }
+}
+
 /// Block error
 #[derive(Clone, Debug, Eq, thiserror::Error, PartialEq)]
 #[non_exhaustive]