easy/chore: support nesting in starting res-api (MystenLabs#14309)

## Description 

Allows specifying where to nest

## Test Plan 

Manual

---
If your changes are not user-facing and not a breaking change, you can
skip the following section. Otherwise, please indicate what changed, and
then add to the Release Notes section as highlighted during the release
process.

### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

crates/sui-rest-api/src/lib.rs

@@ -76,8 +76,13 @@ pub fn rest_router(state: std::sync::Arc<dyn NodeStateGetter>) -> Router {
 pub async fn start_service(
     socket_address: std::net::SocketAddr,
     state: std::sync::Arc<dyn NodeStateGetter>,
+    base: Option<String>,
 ) {
-    let app = rest_router(state);
+    let app = if let Some(base) = base {
+        Router::new().nest(&base, rest_router(state))
+    } else {
+        rest_router(state)
+    };
 
     axum::Server::bind(&socket_address)
         .serve(app.into_make_service())