Lifecycle API

## What problem does this solve or what need does it fill?

Android and XR apps often needs to keep track of the app lifecycle state and react differently to each of them. State-aware behavior is also often needed by backends an plugins implementations.

## What solution would you like?

Some alternatives are:

* `.add_*_system()`, where * is the name of a lifecycle state (or event). This follows the current `.add_startup_system()`.
* `.add_system(my_system.when::())`
* `.add_system_to_schedule(my_system, LifecycleState)`. `.add_system()` will be an alias for the running/steady state systems.

Possible names for the states:

* Startup/OnCreate/Enter
* Resume/OnResume/Resuming
* Update/Running
* Pause/OnPause/Pausing
* OnDestroy/Exit

User resources are available in all states, but engine provided resources might not be available in every state.