Refactor JWT handling in ServiceAccountAccess.

Avoid reading and parsing the private key file on every invocation of
token() in favor or reading it once when the ServiceAccountAccess is
built. Also avoid unnecessary allocations when signing JWT tokens and
renamed sub to subject to avoid any confusion with the std::ops::Sub
trait.

examples/test-svc-acct/src/main.rs

@@ -7,7 +7,9 @@ use yup_oauth2::GetToken;
 async fn main() {
     let creds =
         yup_oauth2::service_account_key_from_file(path::Path::new("serviceaccount.json")).unwrap();
-    let sa = yup_oauth2::ServiceAccountAccess::new(creds).build();
+    let sa = yup_oauth2::ServiceAccountAccess::new(creds)
+        .build()
+        .unwrap();
     let scopes = &["https://www.googleapis.com/auth/pubsub"];
 
     let tok = sa.token(scopes).await.unwrap();