Improve AWS credentials validation

## Description
Consider adding improved validation for the AWS Key and Secret, which warns the user (but does not block the progression through the setup).

Following these specs:
* Verify that the key is 20 chars long and starts with AK.
* Verify that the secret is 40 chars long.

This was implemented but was subsequently removed in (), because AWS does not guarantee that the key and secret will always match this, and the only type of validation currently supported is blocking (which prevents the user from progressing through the setup). The warning validation will let the user know if their key and secret appear to be an incorrect pattern, but will not block the user from progressing through the setup if AWS were to change their key and secret format.