Add client side performance tracking to Posthog

**Is your feature request related to a problem? Please describe.**
We test for performance via CI and track it [here](https://vector-im.github.io/element-x-ios/performance/). It is not possible to catch everything and it would be good to have real world telemetry to help determine where the test suite is incomplete.

**Describe the solution you'd like**
We need real-world telemetry (I assume via posthog). Proposed metrics to track, essentially modelling the CI tests:-
* Launch time
* Login time
* First Sync time
* First Rooms time

**Describe alternatives you've considered**
The only other alternative is to consider serverside stats, but I don't think this will be realistic.

**Additional context**
This issue is motivated by issues like https://github.com/vector-im/element-x-ios/issues/2062 going undetected.