[WIP] Add optional external_id flag when using iam_role

[lebenitza]

When assuming another IAM Role.
More here: https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_create_for-user_externalid.html

I marked this as WIP because I need help updating the tests. Any pointers or actual code is appreciated, I do not usually write much GoLang.

Description

Fixes #2036.

This was manually tested:

• no IAM role to assume
• just the IAM role
• both the IAM role and ExternalId

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added the ability to specify ExternalId when assuming another IAM Role through STS API.