feat: support Azure DevOps as CI backend

[chris-pinola-rf]

🧠 Ai UsageDetails (if applicable):

Used Claude to help orient myself in a new code base and help flesh out the implementation and docs.

---

I'd really like to use OpenTaco, but GitHub Actions is unfortunately not an option right now. I need the compute backend to be Azure DevOps (or even something like Argo Workflows). From what I can tell, supporting a new CI backend is essentially:

• Authenticating to the CI provider
• Resolving a workflow to a pipeline
• Triggering that pipeline with the relevant spec encoded as a parameter

I've been running this patched version of the Orchestrator in my test environment this past week to help me evaluate the platform and it's seemingly working great.

I'm wondering:

1. is this kind of contribution welcome in the project?
2. if so, what's missing? feel free to add to the list below

---

TODO:

• Gracefully fallback to GitHub Actions when DIGGER_CI_BACKEND is unset

Related to #949