Create PoC to demo interfaces and data flow

Goal

The goal is to Implement a POC ("vertical slice") that outlines how the data flow and interfaces will work out between all layers. This will help us understand the general shape of all the pieces we need.

Some code and decisions that should fall out of this:

• Demonstrate shape of new class(es) to replace AvailableServices#access_token for minting tokens. We may need to descope this to just self-signed envs since we need Phase 4 as well to support SM/Dedicated (since it requires syncing in new UP data).
• Demonstrate use of the gitlab-cloud-connector Ruby gem and its DataModel types.
• Demonstrate how to interface with Fulfillment APIs to determine available UPs.
• Demonstrate how the previous 2 points combine into determining token scopes.
• Demonstrate how to automatically use the correct token issuer logic depending on whether self-signed or provided tokens should be used.