`#[derive(WpDerivedRequest)]`'s `contextual_get` should return error for non `SparseFoo` types

[oguzkocer]

contextual_get works the same way as #[derive(WpContextual)] does, as it replaces SparseFoo with FooWithEditContext, FooWithEmbedContext & FooWithViewContext. To ensure that we always work with correct output types, we should return a compiler error if the output type of WpDerivedRequest doesn't have the Sparse prefix if the contextual_get attribute is used.