Support sub-workflows as an operation type

[lodejard]

It becomes difficult very quickly to organize a large workflow into stages with child templates. Specific problems:

• The syntax for including an indented workflow fragment is complicated

    {{# indent depth=4 }}
    {{> child-template-name.yaml }}
    {{/ indent }}

• The same working memory is used for the whole workflow, making it possible to consume values or produce output unexpectedly
• Troubleshooting syntax errors becomes more difficult because the overall result becomes a single, large workflow file.
• Individual stages can't be run directly
• Workflow fragments often have re-usable logic. You end up with a situation where the same fragment file is copied into several locations, leaving you with the problem of keeping the

See Sub-workflows design docs for problem statement and solution proposal