Add Zed Pro Trial Upsell (#29938)

This PR adds an upsell to try Zed Pro

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/client/src/user.rs

@@ -713,6 +713,10 @@ impl UserStore {
         self.current_plan
     }
 
+    pub fn trial_started_at(&self) -> Option<DateTime<Utc>> {
+        self.trial_started_at
+    }
+
     pub fn usage_based_billing_enabled(&self) -> Option<bool> {
         self.is_usage_based_billing_enabled
     }