add_observer(s) should accept a tuple of observers akin to add_systems or add_systems should allow for observers

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

Right now, if you are adding multiple observer "systems" to an app, you must call add_observer for each such "system". This has pretty poor ergonomics IMO.

```rust
app.add_systems(FixedUpdate, (system_a, system_b, system_c)).add_observer(observer_a).add_observer(observer_b).add_observer(observer_c)
```

## What solution would you like?

Either:

```rust
app.add_systems(FixedUpdate, (system_a, system_b, system_c)).add_observers((observer_a, observer_b, observer_c))
```

or:

```rust
app.add_systems(FixedUpdate, (system_a, system_b, system_c, observer_a, observer_b, observer_c))
```

I think I'd likely prefer the latter, though it's less specific, because I could, ostensibly, then also add run_conditions etc to my observers (run if in state, etc).

## What alternative(s) have you considered?

Truthfully, none.