Support using AssumeRole to fetch credentials

[atom-andrew]

The gateway supports fetching credentials when AWS_WEB_IDENTITY_TOKEN_FILE points to a file, but not just using an AssumeRole option with fixed credentials from environment variables.

Describe the solution you'd like
I'd like the gateway to fetch credentials using an AssumeRole request to STS.

Describe alternatives you've considered
None. We don't use WebIdentity since we're not an oath-based client applicaiton, but we do have a role we'd like to assume using fixed credentials.

Additional context
I've been testing out some changes for this at #121. They seem to work.