Unique External ID for Cross Account ARN

We no longer require customers to follow the three-part external ID format. The external ID field does not validate a particular format but instead any combination of alphanumeric characters are accepted.

The only validations present currently are:

•    External Id length should be at least 2 characters.
•    External Id length should not exceed 1024 characters.
•    No space should be present between external Id string.