New Event/Observer API is unfriendly to Entity-like types

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

Hello! I maintain the [`moonshine-kind`](https://github.com/Zeenobit/moonshine_kind) crate, which adds the `Instance` type. The new changes for entity events in Bevy 0.17 make hard assumptions that the underlying target is exactly an `Entity` and nothing else.

This makes implementation of newtype wrappers around `Entity` (like `Instance`) very awkward.

For example:

- The `EntityEvent` trait demands a `&mut Entity` access which forces any `Entity`-like struct to provide mutable entity access. This could be avoided if this trait had a `set_event_target` method instead of `event_target_mut`.
- The `EntityEvent` derive macro expects exactly an `Entity` type. In my opinion, it should work with any [`ContainsEntity`](https://docs.rs/bevy/latest/bevy/ecs/entity/trait.ContainsEntity.html) type.
- The `EntityCommands::trigger` method demands an `impl FnOnce(Entity)` constructor function. There should also be a function that lets you just trigger any `EntityEvent` as-is. I don't see the need to *force* the user to pass a constructor instead of the event itself.

## What solution would you like?

- Refactor `EntityEvent::event_target_mut` to `EntityEvent::set_event_target`
- Support `ContainsEntity` on `#[derive(EntityEvent)]`
- Refactor `EntityCommands::trigger` to take an `impl EntityEvent`, and add `trigger_with` as the ctor version. `trigger_with` should also support `ContainsEntity`.

## What alternative(s) have you considered?

N/A

Start by reading the existing EntityEvent trait, the #[derive(EntityEvent)] macro, and EntityCommands::trigger implementation. Trace how event targets and ContainsEntity are handled, then evaluate the proposed direct-event and constructor-based APIs. Done means entity-like targets are supported without breaking existing event behavior, with the relevant repository tests updated.

Written by the indexing model from the issue text.