Add the concept of reference attributes to the spec

[michaelsafyan]

Fixes #1428

Changes

Adds the concept of "reference attributes" / "reference fields" to the specification.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
• [N/A] schema-next.yaml updated with changes to existing conventions.

[michaelsafyan]

Given the existence of the use of ref in the VCS attributes, I will update this PR with an alternative segment.

[michaelsafyan]

Here are some possible naming alternatives to ref:

• external: given that it will be in external storage
• ext: shorter way of using the name "external"
• foreign: given that it is in a foreign storage location
• remote: given that it may be in another storage location
• ext_storage: similar mash up of the above
• remote_storage: similar mash up of the above
• blob_ref: another possible name

Any other thoughts? Any opinions on what is best?

[michaelsafyan]

I've updated the pull request to use *.blob_ref.* instead of *.ref.* but am open to alternative naming suggestions.

[christophe-kamphaus-jemmic]

I think blob_ref is a good choice.

Should there be some tooling to make sure we don't accidentally define anything blob_ref.* in the future?
Eg. could we add a policy in policies/ to flag a violation if blob_ref.* is detected?

[jsuereth]

NIt: You can provide a policy to enforce this in the /policies directory. Would like to see this happen after this naming and feature is approved.

[michaelsafyan]

Sounds good. I'm happy to either add this enforcement in a separate PR or to add to the existing PR. Would it be better to do that separately or in a single PR?