Update Config::context_providers to use Box instead of Arc

I stumbled across this comment in the code for packages/server/src/Config.rs... just wondering if it's getting (been??) picked up for v0.7??

```rust
pub fn context_providers(mut self, state: ContextProviders) -> Self {
// This API should probably accept Vec Box + Send + Sync + 'static>> instead of Arc so we can
// continue adding to the context list after calling this method. Changing the type is a breaking change so it cannot
// be done until 0.7 is released.
let context_providers = (0..state.len()).map(|i| {
let state = state.clone();
Arc::new(move || state[i]()) as Arc Box + Send + Sync>
});
self.context_providers.extend(context_providers);
self
}
```