Add support for external id when assuming roles

[mcasperson]

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Prerequisites

• [ * ] I have searched open and closed issues to make sure it isn't already requested
• [ * ] I have discussed this in #feature-discussion or #backlog and the consensus is that this is something we plan on adding in the near future
• [ * ] I have written a descriptive issue title
• [ * ] I have linked the original source of this feature request
• [ * ] I have tagged the issue appropriately (area/*, kind/enhancement)

The enhancement

What is the problem this solves or benefit it gives

AWS allows an external ID to be passed when assigning a role: https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_create_for-user_externalid.html. This is something we can support for our AWS steps.

Proposed solution

Expose the external ID field in AWS steps.

Mockup

Workarounds

Roles could be assumed manually in AWS script steps, but other steps like cloudformation deployment don't have a workaround.

Links

https://help.octopus.com/t/feature-request-add-support-for-aws-iam-role-w-third-party-external-id-access/25142

[mcasperson]

This has been merged into master and will be available in 2020.3.0.

[octoreleasebot]

Release Note: External ID now available when assuming an IAM role