`#[reflect(Trait)]` on a generic struct can't conditionally reflect depending on generic parameters

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

Typically a derive macro such as `PartialEq` is clever enough to only work when a struct's generic parameters implement `PartialEq`, by adding trait bounds to the impl. It would be nice to do something similar with `Reflect`:

```rust
#[derive(Reflect, PartialEq)]
#[reflect(PartialEq)] // error
struct Foo {
foo: T,
}
```
Looking at the proc macro it's not immediately obvious to me how do this.

## What solution would you like?

Some way of telling the `Reflect` macro to only reflect traits when the generic type implements them.

## Additional context

This came up in https://github.com/bevyengine/bevy/pull/13348#discussion_r1597879388 When resolved, open a trivial PR to add the `PartialEq` reflection.