Implement `Plugin` for `Box<dyn Plugin>`

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

`DynamicPlugin`s cannot currently be added directly to an `App`, since they are passed across shared library boundaries as `Box`, which does not implement `Plugin`. In short, a simple use case of `bevy_dynamic_plugin` like this should work (ignoring the library's lifetime for demo purposes):

```rust
use bevy::prelude::*;

struct MyModLoaderPlugin{
mod_library: PathBuf,
}

impl MyModLoaderPlugin {
fn build(&self, app: &mut App) {
let (library, plugin) = unsafe { bevy_dynamic_plugin::dynamically_load_plugin(&self.mod_library) }?;
app.add_plugins(plugin);
// Store the library somewhere permanent so it doesn't get unloaded...
}
}
```

## What solution would you like?

Other than some intricacies unknown to me, this should work:

```rust
impl Plugin for Box {
fn build(&self, app: &mut App) {
self.as_ref().build(app);
}

fn ready(&self, app: &App) -> bool {
self.as_ref().ready(app)
}

fn finish(&self, app: &mut App) {
self.as_ref().finish(app);
}

fn cleanup(&self, app: &mut App) {
self.as_ref().cleanup(app);
}

fn name(&self) -> &str {
self.as_ref().name()
}

fn is_unique(&self) -> bool {
self.as_ref().is_unique()
}
}
```

## What alternative(s) have you considered?

I am currently using a wrapper struct to effectively provide the above implementation while respecting trait implementation orphaning rules. It works, but it's unnecessary boilerplate.