Use composite oauth token for Duo Workflows CI execution

What does this MR do and why?

Summary of Changes

This merge request adds support for service accounts to execute Duo Workflows in CI/CD pipelines. The key changes include:

1. Modifying the workflow policy to allow service accounts to access and update workflows they don't own
2. Creating a new service that generates composite OAuth access tokens for workflows running in CI
3. Updating the API to use these composite tokens instead of regular OAuth tokens based on the use_service_account parameter.

References

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• Related to #515422 (closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Onboard duo workflow following steps in !182894 (merged) or via the admin UI.
2. Fetch the Duo Workflow service account user, ::Ai::Setting.instance.duo_workflow_service_account_user .
3. Enable FF Feature.enable(:duo_workflow, service_account_user)
4. Manually add developer access for the project where you want to run Duo Workflow. project.add_developer(service_account_user)
5. Run a CI workflow, following instructions, it should run without any errors. This workflow will use composite identity token for accessing all the GitLab apis and git tools.