Add web identify support

[dkropachev]

What this PR does:
After aws/aws-sdk-go#2667 sdk supports https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRoleWithWebIdentity.html Let's enable it here

Which issue(s) this PR fixes:
Fixes #154

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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PR needs rebase.

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/lifecycle stale

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