Track is_gitlab_team_member for Duo Workflow

Problem

When going towards private and public beta and ultimately GA it becomes more important to distinguish between events created by internal team members and those created by customers. Currently, we can only try to infer this from the namespace Duo Workflow is being triggered in, but this is not scaleable or reliable.

Desired Outcome

is_gitlab_team_member is transferred as a header to Duo Workflow Service so it can be attached to events using the standard context

Proposed solution

Same approach as gitlab-org/duo-workflow/duo-workflow-service#239 (closed)