Add helper functions for AWS Secrets Manager

[bwhaley]

This introduces three helper functions for AWS Secrets Manager:

• CreateSecretStringWithDefaultKey() - Create a secret using the default KMS key
• GetSecretValue() - Get a secret value by name or ARN
• DeleteSecret() - Delete a secret, either immediately or after the default 30 day recovery window

When used with forceDelete=true to delete immediately, this should be a very low cost operation since secrets cost $0.40 per month and they are prorated based on the number of hours.

[bwhaley]

I've incorporated all the feedback. Upon further reflection, given the contribution guidelines, does this really pass the "complexity" test for what should be included in Terratest? As you can see from the functions, nothing here is very complex. Just more convenient. It's fine if you don't think this should be added in order to reduce sprawl... just let me know and I'll move this functionality over to the test I'm working on. Thanks!

[brikis98]

I think this one doesn't do very well on the complexity test, but does do well on the popularity test. I've had to write nearly identical logic several times in our codebase already, and I suspect as we use AWS Secrets Manager more, it'll only show up more and more, so I think I'm OK with adding this.

[bwhaley]

Sounds good. Thanks for the review!