Add streaming subscription for all ethexe promises to RPC

**Problem**
`ethexe-rpc` has no subscription endpoint for promises, so clients must poll rather than stream.

**Proposed solution**
Implement a `subscribe_promises()` streaming endpoint:
```rust
let mut sub = client.subscribe_promises().await?;
while let Some(promise) = sub.next().await {
    handle(promise);
}
```

**Alternatives**
Polling endpoint — simpler but higher latency and load.
