fix 'subject' name in service-account claim

As indicated by
https://developers.google.com/identity/protocols/oauth2/service-account#httprest
the name for the subject-field inside the JWT claim needs to be named
'sub' (instead of 'subject').
This is relevant e.g. for the GSuite Admin Directory API.

src/service_account.rs

@@ -92,6 +92,7 @@ struct Claims<'a> {
     aud: &'a str,
     exp: i64,
     iat: i64,
+    #[serde(rename = "sub")]
     subject: Option<&'a str>,
     scope: String,
 }