Allow not using AWS credentials

[adamnovak]

Julian M has some workflows that want to take S3 URLs as input, but he has AWS credentials that require MFA to use, and he wants to run a lot of workflows at once and not MFA in for each of them.

The files are public; you could get them with aws s3 cp --no-sign-request.

We should add a way for Toil to access S3 URLs without signing its requests and without actually trying to assume any configured AWS roles.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1698

[adamnovak]

Julian says he needs this this week.

[juklucas]

@adamnovak actually it's Julian M who needs this, and I think it's ok if this doesn't happen this week.

[adamnovak]

Oh no I've confused my Julians! Sorry to both of you!