feat: use default provider chains to enable support for sts assume

[jmt-lab]

Issue #, if available: #213

Description of changes:
Similar to awslabs/tough#670
Adds the use of default credential chain and default region chains to enable support for using sts assume role through standard means such as the ~/.aws/config file

Testing Done
Created a build and then setup a role in another account and used a profile defined like

[profile sts-test]
role_arn = ...

I then used my build to download a snapshot using coldsnap --profile sts-test download ...

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[jmt-lab]

Further context on testing:

• Tested downloading snapshot in my account in region
• Tested downloading snapshot in my account cross region
• Tested downloading snapshot in another account via sts assume in region
• Tested downloading snapshot in another account via sts assume cross region

[rpkelly]

With your testing, you were using configurations (other than the sts one, that's cool to add) that would have previously worked, right?

[jmt-lab]

With your testing, you were using configurations (other than the sts one, that's cool to add) that would have previously worked, right?

Yes, i did a bunch of normal use downloads including no aws/config just using IMDS on my ec2 instance

[peterwaller-arm]

Ping. Would be good to be able to use this.