Add an example client to the `reflection` example

## Feature Request

### Crates

`examples`

### Motivation

As a Rust newbie, it took a little bit of time to grok how to set up the `ServerReflectionClient` that's generated in `tonic-reflection`, so having an example would be nice.

### Proposal

Add a simple example for using the client (I have one ready for a PR).

### Alternatives

I don't see a downside to having it documented, but definitely open to changing the example I came up with.