epic: Native LLB refactor of Blubber
We have been dogfood-ing the native-LLB refactor of Blubber for a short while, and it seems functional enough to merge into the main branch. Let's do so and cut a new major version of Blubber (1.0.0) to celebrate the evolution of the software from a small YAML-to-Dockerfile transpiling to a full-fledged BuildKit frontend.
Bug: T345458