[request]: Customize user-agent

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**Tell us about your request**
I'd like to customize the user-agent for my application.

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I'm migrating from rusoto where I can add stuff to the user-agent:

```rust
let mut hyper = HttpClient::new()?;
hyper.local_agent(format!(
"QLDB Driver for Rust v{}/QLDB Shell for Rust v{}",
amazon_qldb_driver::version(),
env!("CARGO_PKG_VERSION")
));
```

**Are you currently working around this issue?**
I'm not!

**Additional context**
N/A

**Attachments**
N/A

No repository file or test is identified. Start by locating where the SDK constructs its user-agent and compare that behavior with the Rust rusoto example in the issue. Done means applications have a defined way to customize the user-agent, with coverage for the resulting value.

Written by the indexing model from the issue text.