add role assumption for s3 source

[codevbus]

Adds assumeRole for AWS/S3 Source

[mcastorina]

Looking good! Just one more piece to wire up and an ineffectual assignment to clean up.

[codevbus]

@mcastorina @dustin-decker - This is ready for review. Some contextual notes and possible discussion points:

• I ended up creating a separate function to handle generating a client for roles. It seemed much easier to break that functionality out vs. nesting a bunch of if/else and iteration inside an auth client
• The S3_Unauthenticated type is actually what triggers if I run Trufflehog locally and have AWS credentials set in local files (~/.aws/credentials)
• The S3_Unauthenticated case defaulted to explicit bucket inputs for generating a list of buckets to scan. I duplicated the functionality under the previous case to enumerate all of the buckets the IAM identity has the ability to make ListBuckets API call for.
• Interestingly, if it defaults to the behavior of enumerating all buckets via ListBuckets, it will say scanning bucket and will not throw an error, even if the IAM policy does not grant the GetObject capability.
• If you pass an explicit list of buckets and an explicit list of roles, it will attempted to scan each bucket with both IAM roles.

[mcastorina]

lgtm - nice work!