httpcaddyfile: Add shortcut for proxy hostport placeholder (#4263)

* httpcaddyfile: Add shortcut for proxy hostport placeholder

I've noticed that it's a pretty common pattern to write a proxy like this, when needing to proxy over HTTPS:

```
reverse_proxy https://example.com {
	header_up Host {http.reverse_proxy.upstream.hostport}
}
```

I find it pretty hard to remember the exact placeholder to use for this, and I continually need to refer to the docs when I need it. I think a simple fix for this is to add another Caddyfile placeholder for this one to shorten it:

```
reverse_proxy https://example.com {
	header_up Host {proxy_hostport}
}
```

* Switch the shortcut name

caddyconfig/httpcaddyfile/httptype.go

@@ -113,6 +113,7 @@ func (st ServerType) Setup(inputServerBlocks []caddyfile.ServerBlock,
 		"{tls_client_serial}", "{http.request.tls.client.serial}",
 		"{tls_client_subject}", "{http.request.tls.client.subject}",
 		"{tls_client_certificate_pem}", "{http.request.tls.client.certificate_pem}",
+		"{upstream_hostport}", "{http.reverse_proxy.upstream.hostport}",
 	)
 
 	// these are placeholders that allow a user-defined final