refs: set up integration branch for the reftable backend

This merge request sets up an initial integration branch for the reftable backend. This integration branch is not expected to be in a perfect state yet. Specifically, the intent of this merge request is not to get a review of the reftable implementation, but rather get a review of the surrounding test infrastructure introduced with it. The result of this merge request should be:

• A baseline that has a green CI pipeline. This is required so that we do not inadvertently regress functionality.
• A CI pipeline that exercises both the files refbd and the reftable refdb.
• Tests which fail for the reftable refdb should be annotated as such so that they can fail. Tests which start to pass should result in a notification so that we can notice and demonstrate progress

This branch will then serve as the baseline for collaboration around upstreaming the reftable backend where we can use merge requests as our main tool to land new changes into our local reftable backend. The branch will be regularly rebased by myself against the latest changes master. Furthermore, I will in regular intervals squash changes together: we do not want to send a patch series upstream that introduces a faulty reftable backend first which then gets fixed over a plethora of patches, so the history will need to get lost at some point in time.

Once this branch lands we can thus kick off collaborative development in the reftable backend.

Closes reftable: set up integration branch with green ... (#186 - closed).