Function spawn_rpc_proxy_mapping_method 

pub async fn spawn_rpc_proxy_mapping_method(
    endpoint: String,
    method: &'static str,
    map: impl Fn(&Value, Value) -> Value + Send + Sync + 'static,
) -> String

Spawns an RPC proxy that forwards every request upstream and passes each method result, together with the request params, through map before returning it.