Automatically derive UseContext implementation inside #[cgp_component]

[soareschen]

Summary

This PR enhances the #[cgp_component] macro families to include an auto derivation of implementation of UseContext.

UseContext is used as a provider that simply delegates the implementation back to the context. It is the reverse of consumer-to-provider delegation.

As an example, given the following:

#[cgp_component {
    provider: NameGetter,
}]
pub trait HasName {
    fn name(&self) -> &str;
}

The following UseContext implementation would be derived:

impl<Context> NameGetter<Context> for UseContext
where
    Context: HasName,
{
    fn name(context: &Context) -> &str {
        context.name()
    }
}

The use of UseContext can be useful in the advanced CGP usage of higher-order providers, where providers are passed as type parameters directly to other providers. The auto derivation of UseContext will make it simpler to make use of higher-order provider patterns, without needing to explicitly derive the implementation on all relevant CGP traits.