Verify Sign Blob works outside GDU

[bshaffer]

Ensure that the methods in SignBlobInterface, Iam, and IamSignerTrait work outside the GDU

[bshaffer]

signBlob is implemented in two different places:

1. In the ServiceAccountSignerTrait - the signing key is used to sign the blob directly. No network call is made
2. In the IamSignerTrait - The Iam helper class is used to make an API call to the IAM API in order to sign the blob.

In the case of (1), no network call is made (it's already Universe Domain agnostic). In the case of (2), the IAM object is created with the configured universe domain. Because the universe domain is already supported in Iam::signBlob, this is supported.