Have full control over the User-Agent

## Feature Request

### Motivation

The User-Agent header is populated, by default, with `tonic/x.y.z` (x.y.z. being the current tonic version number) \
There is a [user_agent](https://docs.rs/tonic/latest/tonic/transport/channel/struct.Endpoint.html#method.user_agent) method to somehow customize this by adding a prefix to this `tonic/x.y.z` part.

### Proposal

In some use cases, users could want to _totally_ customize the user-agent. Not suffixing with `tonic/x.y.z` would probably be a good idea.

I'm not sure why this suffix has been made compulsory anyway (but maybe I'm missing something? Is this required by some spec? I feel that any server _requiring_ to know the client version is probably poorly designed)

Thanks