Support read-only calendar store as preparation for birthday calendars

crates/store/src/calendar_store.rs

@@ -61,4 +61,6 @@ pub trait CalendarStore: Send + Sync + 'static {
         cal_id: &str,
         object_id: &str,
     ) -> Result<(), Error>;
+
+    fn is_read_only(&self) -> bool;
 }

crates/store/src/contact_birthday_store.rs

@@ -140,4 +140,8 @@ impl<AS: AddressbookStore> CalendarStore for ContactBirthdayStore<AS> {
     ) -> Result<(), Error> {
         Err(Error::ReadOnly)
     }
+
+    fn is_read_only(&self) -> bool {
+        true
+    }
 }