Add flow collections to the API

[robwadge]

Details

ADR and associated implementation proposing addition of Flow collections to the API, to support multi-essence Flows

Pivotal Story (if relevant)

Story URL: https://www.pivotaltracker.com/story/show/187060669

Related PRs

Where appropriate. Indicate order to be merged.

Submitter PR Checks

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• PR completes task/fixes bug
• API version has been incremented if necessary
• ADR status has been updated, and ADR implementation has been recorded
• Documentation updated (README, etc.)
• PR added to Pivotal story (if relevant)
• Follow-up stories added to Pivotal

Reviewer PR Checks

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• PR completes task/fixes bug
• Design makes sense, and fits with our current code base
• Code is easy to follow
• PR size is sensible
• Commit history is sensible and tidy

Info on PRs

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[samdbmg]

Looks good overall - I've inlined a few nits and a question about creating new Sources

[philipnbbc]

A couple of my comments inline

[philipnbbc]

LGTM. As discussed, there are some complications around grouping conflicts and what to do if Flows get deleted (Source inference conflicts, missing slots, etc.), but that's something we can figure that out as we implement these changes.

I was surprised the the JSON schema validator was ok with the codec and container properties being on flow-core and only required in flow-video etc., but it turns out that is valid according to a JSON schema editor.