Maintenance: Ensure all docs have guidance on how to unit test

[heitorlessa]

We've discovered recently that Idempotency and Batch utilities don't have mechanisms to ease testing for customers - they can't use boto stubber since we don't expose the client easily, nor create our clients lazily.

Today, we've discovered that we weren't accepting a boto3 session in Parameters to ease integration with placebo for testing as well as fetch parameters cross-account.

This issue is to track effort in this area

[heitorlessa]

Closing in favour of the new boto3 session