Provide easy access to components of targeted entity in observers

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

When writing observers, it is frequently desirable to be able to access a component on the listener entity, that is, the entity in the trigger event.

Currently there are two ways to do this: either inject a query for all components of that type and pick the one you want; or inject a DeferredWorld, and then look up the component you need on the event's entity.

bevy_mod_picking had convenience methods for injecting components directly. I'd like to see something similar for Bevy.

## What solution would you like?

I'm not sure about naming, but I was thinking of something like:

```rust
entity.observe(trigger: Trigger>, component: ListenerRef);
```
There would be a `ListenerMut` version as well. Alternate names might be `TargetRef`, `TriggerRef`, or `ObserverEntityRef`.

## What alternative(s) have you considered?

For now, I'm using DeferredWorld, as I am unsure about the overhead of injecting a Query.

## Additional context

bevy_mod_picking also provided a means to access the "target" entity and it's components, which was the entity upon which the event was triggered before bubbling, not the entity which handled the event. However, this feature was used rarely (at least by me anyway).

This is also somewhat related to a previous ticket filed by me a long time ago (#11048) - that was rejected (for good reason), but this ticket provides a much more narrowly-scoped variation of the same idea.

Inizia esaminando le API esistenti Observer, Trigger, Query e DeferredWorld di Bevy, quindi confronta il modello di accesso proposto con la issue precedente #11048 e i metodi di convenienza di bevy_mod_picking. Il lavoro sarà considerato completato quando sarà disponibile un’API concordata per l’accesso immutabile e mutabile ai componenti dell’entità listener, con il relativo design e ambito definiti.

Scritto dal modello di indicizzazione a partire dal testo della issue.