Add SystemClock (#8239)

This PR adds a `SystemClock` trait for abstracting away the system
clock.

This allows us to swap out the real system clock with a
`FakeSystemClock` in the tests, thus allowing the fake passage of time.

We're using this in `Telemetry` to better mock the clock for testing
purposes.

Release Notes:

- N/A

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -385,6 +385,7 @@ impl Render for MessageEditor {
 mod tests {
     use super::*;
     use client::{Client, User, UserStore};
+    use clock::FakeSystemClock;
     use gpui::TestAppContext;
     use language::{Language, LanguageConfig};
     use rpc::proto;
@@ -455,8 +456,9 @@ mod tests {
             let settings = SettingsStore::test(cx);
             cx.set_global(settings);
 
+            let clock = Arc::new(FakeSystemClock::default());
             let http = FakeHttpClient::with_404_response();
-            let client = Client::new(http.clone(), cx);
+            let client = Client::new(clock, http.clone(), cx);
             let user_store = cx.new_model(|cx| UserStore::new(client.clone(), cx));
             theme::init(theme::LoadThemes::JustBase, cx);
             language::init(cx);