fix(graphql): restore --node-rpc-url flag (#19710)

## Description

I accidentally turned it into a positional argument in a recent
refactoring. This change changes it back.

## Test plan

```
sui$ cargo run --bin sui-graphql-rpc -- start-server --help
```

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API:

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

@@ -219,6 +219,7 @@ pub struct InternalFeatureConfig {
 #[derive(clap::Args, Default)]
 pub struct TxExecFullNodeConfig {
  /// RPC URL for the fullnode to send transactions to execute and dry-run.
+ #[clap(long)]
  pub(crate) node_rpc_url: Option<String>,
 }