Add Basic Concurrent Example for S3

[darklight3it]

📬 Issue #, if available:

✍️ Description of changes:

Adding a basic concurrent example for S3 call case.

🔏 By submitting this pull request

• I confirm that I've ran cargo +nightly fmt.
• I confirm that I've ran cargo clippy --fix.
• I confirm that I've made a best effort attempt to update all relevant documentation.
• [x ] I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jlizen]

looks good

might also be nice to have a more generalized 'shared state' example that has eg:

struct SharedState {
    my_expensive_to_clone_value: T,
    another_expensive_to_clone_value: T
}

async fn main {
...

let state = Arc::new( SharedState { .. });

 lambda_runtime::run_concurrent(service_fn(move |event: LambdaEvent<Request>| {
   let state = state.clone();
   async move {
       ..
   }
}

[jlizen]

@darklight3it were you intentionally waiting to merge?

[darklight3it]

No, last week I got carried away by other stuff. I'll try to have a look at this tomorrow.