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/workspace/Cargo.toml

@@ -25,6 +25,7 @@ async-recursion = "1.0.0"
 bincode = "1.2.1"
 call.workspace = true
 client.workspace = true
+clock.workspace = true
 collections.workspace = true
 db.workspace = true
 derive_more.workspace = true