Add DataContext trait

[smmoosavi]

Define the DataContext trait and implement for ContextBase and ExtensionContext

Why

Sometimes we have a function that gets Context as an argument, we can't use that function in Extension

pub fn get_foo_services(ctx: &async_graphql::Context<'_>) -> impl FooServices {}

#[async_trait::async_trait]
impl Extension for FooExtension {
    async fn execute(
        &self,
        ctx: &ExtensionContext<'_>,
        operation_name: Option<&str>,
        next: NextExecute<'_>,
    ) -> Response {
        let result = next.run(ctx, operation_name).await;
        let services = get_foo_services(ctx);

        result
    }
}

         let services = get_hsm_services(ctx);
                                         ^^^ expected struct `async_graphql::ContextBase`, found struct `async_graphql::extensions::ExtensionContext`

[sunli829]

This is very useful, thank you! 🙂