Blueprint: GitLab Steps - Iterate on technical strategy

The following discussion from !124601 (merged) should be addressed:

• @josephburnett started a discussion:

  After applying a few suggestions, I think we should merge this as-is. The Summary establishes some good boundaries and I think we should work on the Proposal in another MR.

  In particular @ayufan and I have agreed to split some of the work, so I would like to submit an MR for my part. This problem can be divided into two main areas:

  1. Compiling GitLab CI language and GitHub Actions into GitLab Steps. Kamil proposes this happens on the GitLab side, but it could happen in the first layers of the Runner as well. Kamil will take on this part.
  2. My work will be to define a basic GitLab Steps syntax (in proto) and to build the Step Runner and the data pipeline through Runner. My first contribution to the blueprint will be a high level description of the data flow and a few diagrams. I think it's important to explain (visually as well as in text) what is different between how the runner works now, and how it will work with Steps.

  I actually explain some of what the difference will be in this overview: https://youtu.be/nZoO547IISM (28 minutes)