Use var o environment vars in service connectio external-id value

**Describe the solution you'd like**
For security and audit purpose the ability to substitute the external-id value in the service connection with a variable value so we can pass to AWS Cloudtrail service what user, commit or any other useful value from the pipeline environment.

The issue names no files, tests, or entry points. Start by tracing how Azure DevOps service connections produce the external-id sent to AWS CloudTrail; done means the value can be populated from a pipeline environment variable and the resulting value is available for audit purposes.

Written by the indexing model from the issue text.