Create ADR on customization for Duo Workflow

Context

Going forward, rather than just offering one multi-purpose workflow, as is currently the case in the IDE, we'll want to allow everybody to more easily develop, run and observe purpose built Workflows (examples in &16213).

It is already possible to create additional workflows internally as can be seen in https://gitlab.com/gitlab-org/duo-workflow/duo-workflow-service/-/issues/327, but there is still a lot of understanding involved.

To allow for this we'll want to first enable developers to create workflows more easily internally, but then as fast as possible move towards making it easier and easier to create new workflows and customize existing ones to the point that customers can do the customization and building of workflows themselves.

Desired Outcome

We need a clear architectural design direction on how the customization of Workflows should look like in the future, to allow for the parallel development of both the actual layer of customization as well as surrounding services, such as hosting the different workflow/agent configurations, discoverability through a catalog, editing workflows/agents.

Requirements

Requirements are outlined in &17574.