RPC support for SSE

### What is the feature you are proposing?

We require using EventSource to work with SSE routes, but can we have Hono's RPC to have this functionality as well?

```ts
const client = hc<typeof app>(`https://${BASE_URL}`)
client.sse.$get({
  onMessage(event) { /* do something with this */ },
  // other callbacks
}, {
  headers: {
    Authorization: 'Bearer booyah'
  }
})
```