Support managed session polices for STS via PolicyArns parameter

[timoguin]

Is this related to a problem?

STS managed session policies are unavailable to any consumers of the SDK.

Feature description

All the STS assume role operations now allow passing IAM session policies that can further scope down the permissions of the session. I'd like to see support for the PolicyArns argument for those APIs.

Describe alternatives you've considered

There are none.

Additional context

References:

• https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRole.html
• https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRoleWithSAML.html
• https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRoleWithWebIdentity.html
• https://docs.aws.amazon.com/STS/latest/APIReference/API_GetFederationToken.html
• https://docs.aws.amazon.com/IAM/latest/UserGuide/access_policies.html#policies_session
• https://aws.amazon.com/blogs/security/create-fine-grained-session-permissions-using-iam-managed-policies/

[timoguin]

I finally found the PolicyArns arguments in the docs: https://docs.aws.amazon.com/sdk-for-go/api/service/sts/#AssumeRoleInput.SetPolicyArns

Closing this out since it seems to already be supported!