rpc: Tx projections

StreamingTx should have a method like `as_event`, with roughly following API (trait bounds and errors omitted):
```rust
impl StreamingTx {
pub fn as_event_sender(&mut self) -> EventSender;
}

pub struct EventSender<'a, E>(&'a mut StreamingTx);

impl EventSender{
pub async fn send_event(&mut self, ev: E);
// note - no `finish` method
}

```