[type safety] add type enforcement for SubscriptionSink?

[devtempmac]

Current rpc definition allows adding types to item:

#[subscription(name = "subscribeStorage" => "override", item = Vec<Hash>)]
    async fn subscribe_storage(&self, keys: Option<Vec<StorageKey>>) -> SubscriptionResult;

but this isn't enforced at the server-side:

async fn subscribe_storage(
        &self,
        pending: PendingSubscriptionSink,
        _keys: Option<Vec<ExampleStorageKey>>,
    ) -> SubscriptionResult {
        let sink = pending.accept().await?;
        let json = serde_json::value::to_raw_value(&vec![[0.123123; 32]])?; // compiles ok (can send anything)
        sink.send(json).await?;

        Ok(())
    }

Can this be enforced with SubscriptionSink by adding a generic param?

On the serverimplementation side, it would be:

async fn subscribe_storage(
        &self,
        pending: PendingSubscriptionSink<Hash>,
        _keys: Option<Vec<ExampleStorageKey>>,
    ) -> SubscriptionResult {
        let sink = pending.accept().await?;
        // ...
        sink.send(not_hash).await?; // <-- compiler type error
        sink.send(some_hash).await?; // <-- compiler ok

        Ok(())
    }