Adding an IntoResponse trait

## Feature Request

### Crates

tonic-build

### Motivation

When using a generated client, you can pass either a `Message` or a `Request`, as the client accepts `impl IntoRequest`. This makes things more ergonomic when you don't have any headers/options to attach.

The generated server code lacks a similar IntoResponse and IntoStreamingResponse trait, so all responses need to be wrapped in Response::new(). This makes things a bit more cumbersome, and inconsistent with the client side of things (I remember being a bit confused by this when I started using Tonic).

Are there any technical limitations or other reasons that would prevent adding such functionality in the next breaking update?

(Apologies if this has been discussed before. I found a brief mention on https://github.com/hyperium/tonic/pull/66#issuecomment-545213939 but couldn't see an issue for this specifically)

### Proposal

Adding a similar trait:

```rust
pub trait IntoResponse {
fn into_response(self) -> Response;
}

impl IntoResponse for T {
fn into_response(self) -> Response {
Response::new(self)
}
}

impl IntoResponse for Response {
fn into_response(self) -> Response {
self
}
}

async fn some_method(...) -> Result, tonic::Status> {
Ok(Response::new(SomeMessage {}))
}

async fn some_method(...) -> Result, tonic::Status> {
Ok(SomeMessage {})
}
```

prost-build would presumably need to be updated to change the signature of the generated methods, and call .into_response() inside call().

One potential downside is increased compile time due to the extra type inferences, but such a cost is already paid when compiling the client.

If this is something you'd be amenable to, I can look into it further.

### Alternatives

https://github.com/hyperium/tonic/pull/1064 would continue to require manual wrapping, but would reduce the keystrokes required to do so.