Using tokio for parallelism & async

This section covers how to utilize async functions with `tokio`.

Example use case: calling an API endpoint to get many requests in parallel.

Structure:

1. Define a response struct with serde
1. Create an async function to query the endpoint using async reqwest
2. Wrapper function to spawn many tasks in parallel second example (https://docs.rs/tokio/latest/tokio/task/fn.spawn.html#examples?)
3. create an extendr function with a runtime to evaluate the async function in a blocking context.

This can be taught in a progressive manner as individual sections