[GraphQL] generate-config sub-command

## Description

Add a command for generating a `config.toml` file for the GraphQL
service with all its parameters set to their default values.

## Test plan

```
cargo run --bin sui-graphql-rpc -- generate-config /tmp/config.toml
```

crates/sui-graphql-rpc/src/commands.rs

@@ -13,6 +13,13 @@ use std::path::PathBuf;
     version
 )]
 pub enum Command {
+    /// Output a TOML config (suitable for passing into the --config parameter of the start-server
+    /// command) with all values set to their defaults.
+    GenerateConfig {
+        /// Optional path to a file to output to. Prints to stdout if none is provided.
+        output: Option<PathBuf>
+    },
+
     StartServer {
         /// The title to display at the top of the page
         #[clap(short, long)]

crates/sui-graphql-rpc/src/main.rs

@@ -37,6 +37,19 @@ static VERSION: Version = Version {
 async fn main() {
     let cmd: Command = Command::parse();
     match cmd {
+        Command::GenerateConfig { output } => {
+            let config = ServiceConfig::default();
+            let toml = toml::to_string_pretty(&config).expect("Failed to serialize configuration");
+
+            if let Some(path) = output {
+                fs::write(&path, toml).unwrap_or_else(|e| {
+                    panic!("Failed to write configuration to {}: {e}", path.display())
+                });
+            } else {
+                println!("{}", toml);
+            }
+        }
+
         Command::StartServer {
             ide_title,
             db_url,

crates/sui-graphql-rpc/src/server/graphiql_server.rs

@@ -31,7 +31,7 @@ pub async fn start_graphiql_server(
     version: &Version,
     cancellation_token: CancellationToken,
 ) -> Result<(), Error> {
-    info!("Starting server with config: {:?}", server_config);
+    info!("Starting server with config: {:#?}", server_config);
     info!("Server version: {}", version);
     start_graphiql_server_impl(
         ServerBuilder::from_config(server_config, version, cancellation_token).await?,