ssbc: Propose GraphQL schema

Created by: eseliger

TODOS:

• Doesn't support easily listing the skipped workspaces in the same list
  • Force execution on single workspace that was excluded
    • Instead of forcing single ones, do we want to move allowIgnored into the spec?
• Stats for batch spec workspaces, for top of page stats row
• Support recent diffs, recent errors
• Clear cache option on createFromRaw (maybe on execute, too?)
  • On both for preview, number of steps is different without cache. Want to be able to preview that, too.
• Filtering, sorting on BatchSpec.workspaces
• Retry all (exact same workspaces), retrySelection

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• Maybe: Read only flag for batch spec?
• What about tracking changesets