Affinitize System Sets to Schedules

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

Sometimes people do things like:

```rust
#[derive(SystemSet, Debug, Clone, Copy, PartialEq, Eq, Hash)]
struct X;

app
.configure_sets(Update, X.run_if(in_state(GameState::Playing)))
.add_systems(Update, foo.in_set(X))
.add_systems(PostUpdate, foo.in_set(X))
```

Then they get confused when `foo` runs unconditionally in `PostUpdate` (because sets are "per schedule" configuration).

## What solution would you like?

One way to ensure this never happens is to require a specified ScheduleLabel type when implementing SystemSet.

```rust
#[derive(SystemSet, Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[system_set(Update)]
struct X;

app
.configure_sets(Update, X.run_if(in_state(GameState::Playing)))
.add_systems(Update, foo.in_set(X))
// This fails to compile
.add_systems(PostUpdate, foo.in_set(X))
```

I haven't fully thought this one through yet / I'm not sure we _actually_ want this solution. Just putting the idea out there.