It should be _easy_ to detect when a stream ends

## Feature Request

Simply put, when I have stream, I may need to do cleanup when it is closed. Everything else is a memory leak.

There are multiple issues that discuss this, most notably https://github.com/hyperium/tonic/issues/377

The solution proposed in the referenced issue is far from trivial. I don't consider replacing low level details related to future impls a sufficient solution for a library at the level of tonic. While I will be forced to get down to that level in my app, I am fairly certain that the API this should be providing is one of an enum, say

```rust
enum StreamEvent {
Message(T),
ClientDisconnect,
// the following is more of a guess
Error(E),
}
```

This feels quite natural to me, and so I wonder why it isn't already done this (or a similar) way. Are there technical difficulties to detect connection termination? Any other "hard" reason? Or was this just never tackled?

### Motivation

It is of utmost relevance to clean up resources that are not needed anymore

### Proposal

See above

### Alternatives

One could track connections via IDs and have a separate event that tells us which ones have terminated, but I feel that would increase complexity