feat: update Rust template (netlify#6314)

The handler_fn function was deprecated in 0.5.0 of the lambda_runtime in favour of the service_fn.

src/functions-templates/rust/hello-world/src/main.rs

@@ -1,21 +1,21 @@
 use aws_lambda_events::event::apigw::{ApiGatewayProxyRequest, ApiGatewayProxyResponse};
 use aws_lambda_events::encodings::Body;
 use http::header::HeaderMap;
-use lambda_runtime::{handler_fn, Context, Error};
+use lambda_runtime::{service_fn, Error, LambdaEvent};
 use log::LevelFilter;
 use simple_logger::SimpleLogger;
 
 #[tokio::main]
 async fn main() -> Result<(), Error> {
     SimpleLogger::new().with_utc_timestamps().with_level(LevelFilter::Info).init().unwrap();
 
-    let func = handler_fn(my_handler);
+    let func = service_fn(my_handler);
     lambda_runtime::run(func).await?;
     Ok(())
 }
 
-pub(crate) async fn my_handler(event: ApiGatewayProxyRequest, _ctx: Context) -> Result<ApiGatewayProxyResponse, Error> {
-    let path = event.path.unwrap();
+pub(crate) async fn my_handler(event: LambdaEvent<ApiGatewayProxyRequest>) -> Result<ApiGatewayProxyResponse, Error> {
+    let path = event.payload.path.unwrap();
 
     let resp = ApiGatewayProxyResponse {
         status_code: 200,