Support AWS assume_role external id

[wiseelf]

It would be cool if we could assume role with external id set. Something like this:

  - path_regex: secrets/environments/production/secrets.yaml
    kms: 'arn:aws:kms:us-east-1:666666666:key/xxxx-x-xxxxx+arn:aws:iam:us-east-1:666666666:role/kms_role'
    external_id: 'some_secret_external_id'

or

creation_rules:
  - kms: arn:aws:kms:us-east-1:666666666:key/xxxx-x-xxxxx+arn:aws:iam:us-east-1:666666666:role/kms_role+some_secret_external_id

What do you think?

[wiseelf]

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