Listen on any IPv4 or any IPv6 address. (#428)

## Description

Listening on `0.0.0.0` only binds to IPv4 addresses. We can listen on
IPv4 and IPv6 by using `::`.

I have opted to use the constants for this rather than parsing a string,
both for clarity and to avoid errors.

## Changelog

- Add a changelog entry (in the "Changelog entry" section below) if the
changes in this PR have any user-facing impact. See [changelog
guide](https://github.com/hasura/graphql-engine-mono/wiki/Changelog-Guide).
- If no changelog is required ignore/remove this section and add a
`no-changelog-required` label to the PR.

### Product
_(Select all products this will be available in)_
- [x] community-edition
- [ ] cloud
<!-- product : end : DO NOT REMOVE -->

### Type
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https://github.com/hasura/graphql-engine-mono/wiki/Changelog-Guide#structure-of-our-changelog
-->
_(Select only one. In case of multiple, choose the most appropriate)_
- [ ] highlight
- [x] enhancement
- [ ] bugfix
- [ ] behaviour-change
- [ ] performance-enhancement
- [ ] security-fix
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### Changelog entry
<!--
  - Add a user understandable changelog entry
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The v3 engine now binds to all IPv4 and IPv6 addresses. Previously it
only used IPv4.

<!-- changelog-entry : end : DO NOT REMOVE -->

<!-- changelog : end : DO NOT REMOVE -->

V3_GIT_ORIGIN_REV_ID: d63b5544bfbf2ad067113980fa61dd74213c7b78

v3/crates/engine/bin/engine/main.rs

@@ -1,4 +1,5 @@
 use std::fmt::Display;
+use std::net;
 use std::path::PathBuf;
 use std::sync::Arc;
 
@@ -26,6 +27,8 @@ use hasura_authn_jwt::jwt;
 use hasura_authn_webhook::webhook;
 use lang_graphql as gql;
 
+const DEFAULT_PORT: u16 = 3000;
+
 #[derive(Parser)]
 #[command(version = VERSION)]
 struct ServerOptions {
@@ -36,7 +39,7 @@ struct ServerOptions {
     #[arg(long, value_name = "AUTHN_CONFIG_FILE", env = "AUTHN_CONFIG_PATH")]
     authn_config_path: PathBuf,
     #[arg(long, value_name = "SERVER_PORT", env = "PORT")]
-    port: Option<i32>,
+    port: Option<u16>,
 }
 
 struct EngineState {
@@ -172,14 +175,16 @@ async fn start_engine(server: &ServerOptions) -> Result<(), StartupError> {
         .merge(explain_route)
         .merge(health_route);
 
-    let addr = format!("0.0.0.0:{}", server.port.unwrap_or(3000));
-
-    let log = format!("starting server on {addr}");
+    // The "unspecified" IPv6 address will match any IPv4 or IPv6 address.
+    let host = net::IpAddr::V6(net::Ipv6Addr::UNSPECIFIED);
+    let port = server.port.unwrap_or(DEFAULT_PORT);
+    let address = net::SocketAddr::new(host, port);
+    let log = format!("starting server on {address}");
     println!("{log}");
     add_event_on_active_span(log);
 
     // run it with hyper on `addr`
-    axum::Server::bind(&addr.as_str().parse().unwrap())
+    axum::Server::bind(&address)
         .serve(app.into_make_service())
         .with_graceful_shutdown(shutdown_signal())
         .await