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/zed/Cargo.toml

@@ -34,6 +34,7 @@ channel.workspace = true
 chrono = "0.4"
 cli.workspace = true
 client.workspace = true
+clock.workspace = true
 collab_ui.workspace = true
 collections.workspace = true
 command_palette.workspace = true