Improve ergonomics when passing Clients as function arguments

## Feature Request

### Motivation

At the moment, any function that takes a Client as a parameter requires specifying some pretty scary-looking type bounds:

```rust
async fn foo(mut client: MyClient)
where
T: tonic::client::GrpcService,
T::ResponseBody: tonic::codegen::Body + tonic::codegen::HttpBody + Send + 'static,
T::Error: Into,
::Error: Into + Send,
::Data: Into + Send,
{ ... }
```

It would be nice to somehow simplify these sorts of declarations.

### Proposal

I'm not entirely sure how to work around this issue!

A heavyweight solution might be to write a proc macro that automagically includes these type bounds on a function. e.g:

```rust
#[tonic::clientbounds(T)]
async fn foo(mut client: MyClient) { ... }
```

### Alternatives

There are probably other, simpler solutions, but I can't think of any great ones off the top of my head. That said, I don't have a thorough understanding of tonic internals, so maybe there is an easy fix.