Added get_items_by_url, to fetch multiple items at once

src/traits.rs

@@ -83,6 +83,10 @@ pub trait DavCalendar : BaseCalendar {
     /// Returns a particular item
     async fn get_item_by_url(&self, url: &Url) -> Result<Option<Item>, Box<dyn Error>>;
 
+    /// Returns a set of items.
+    /// This is usually faster than calling multiple consecutive [`get_item_by_url`], since it only issues one HTTP request.
+    async fn get_items_by_url(&self, urls: &[Url]) -> Result<Vec<Option<Item>>, Box<dyn Error>>;
+
     /// Delete an item
     async fn delete_item(&mut self, item_url: &Url) -> Result<(), Box<dyn Error>>;