Add sourceArn to sts through headers

[haoranleo]

What this PR does / why we need it:
Add cluster details (clusters_arn and account_id) as headers to STS when assuming a role to make requests. When sourceARN is not empty, it will panic if it fails to parse account id from the sourceARN (same behavior as verifying roleARN).

The reason to add sourceArn and sourceAccount to STS request headers is to avoid confusion deputy issue. AWS IAM supports these global conditional key in IAM roles trust policy https://docs.aws.amazon.com/IAM/latest/UserGuide/confused-deputy.html, and we want to include the caller identity into the STS requests when assuming the role. We have did same change for CCM in kubernetes/cloud-provider-aws#649

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[k8s-ci-robot]

Hi @haoranleo. Thanks for your PR.

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[bryantbiggs]

/ok-to-test

[haoranleo]

Thanks @bryantbiggs, could you review when you have a chance?

[bryantbiggs]

code wise things look good - but I am not an IAM expert so I'll let Micah or someone else weigh in for final approval

/lgtm

[micahhausler]

I don't think we can merge this. The additional header is not signed, and can be changed by a middleman. If you did add it to the signed header, thats not backward compatible. We do have an existing scoping/aud check with the cluster ID

[micahhausler]

/hold

[micahhausler]

Ok, I was misunderstanding, this is about the server side role assumption for EC2 instance verification. Yea I think this check is probably fine to add.

[haoranleo]

@micahhausler would you like to review when you have a chance?

[micahhausler]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: haoranleo, micahhausler

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[haoranleo]

@micahhausler could you remove the hold label?

Also PR on release branch #751

[micahhausler]

/unhold