Unclear how to get request host

## What is the problem your feature solves, or the need it fulfills?

This is code I have

```
async fn upstream_peer(
&self,
session: &mut Session,
_ctx: &mut Self::CTX,
) -> pingora::Result> {
let Some(host) = session.req_header().uri.host() else {
return Err(pingora::Error::new_str("host unset"));
};
```

it reports host unset. Which is probably expected. But then how do I get host? I guess

```
session.req_header().headers.get("host")
```

or

```
session.req_header().headers.get(":authority")
```

or both?

## Describe the solution you'd like

Add a function like

```
RequestHeader::host(&self) -> &str
```

## Describe alternatives you've considered

Continue reading sources.

## Additional context

0.5.0