Use standard CloudConnector header fields in Workflow client

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Problem

A discussion surfaced that we should not hand-craft Cloud Connector header fields like this. We have abstractions in place to inject these: https://docs.gitlab.com/ee/development/cloud_connector/#access-token

The problem with this approach here is that we might accidentally miss injecting important headers. For example, we now require the X-Gitlab-Duo-Seat-Count header to be present for purposes of rate limiting.

Desired Outcome

We should use Gitlab::CloudConnector.ai_headers instead.