add remote PDF fetcher with caching

src/main.rs

@@ -1,9 +1,13 @@
 mod extract;
+mod fetch;
 
 use extract::DataSet;
+use fetch::PdfFetcher;
 
-fn main() {
-    let doc = pdf::file::File::open("data.pdf").expect("Failed to read PDF");
+#[tokio::main]
+async fn main() {
+    let mut fetcher = PdfFetcher::new().expect("Failed to initialize PDF fetcher");
+    let doc = fetcher.fetch().await.expect("Failed to fetch PDF");
     let dataset = DataSet::extract(&doc).expect("Failed to extract dataset");
     for row in dataset.csv_rows() {
         println!("{}", row.unwrap());