EntityEvent cannot be derived for enums

## Bevy version and features
0.17.2

## What you did
Porting to 0.17, where I previously had an `Event` used as an entity event for an enum.

```rust
#[derive(EntityEvent)]
enum A {}
```

## What went wrong

> EntityEvent can only be derived for structs.

Start at the EntityEvent derive entry point and reproduce the reported diagnostic with `#[derive(EntityEvent)] enum A {}`. Compare the enum case with the currently supported struct case and add coverage for the intended enum behavior. Done means the enum no longer produces the reported struct-only error and the relevant tests pass.

Written by the indexing model from the issue text.