Move code to be shared between Steps and to suitable location

Move code to be shared between Steps and Runner to suitable location.

• New location could be entirely new project/repo, or in this project (TBD which of these we choose).
  • The latter would probably require creating a second go module https://gitlab.com/gitlab-org/step-runner/ to hold only the common/shared code.

Current Step modules that should move into this location:

• https://gitlab.com/gitlab-org/step-runner/-/tree/main/proto
• • https://gitlab.com/gitlab-org/step-runner/-/tree/main/schema

Note: some Runner modules will similarly move.

See gitlab!144373 (merged) for dicussion and context.