Define CustomDataDefinition for interactive atoms

[Fweddi]

This updates the interactive atom thrift model to add an optional list of CustomDataDefintion options, opening the door to parameterised interactive atoms.

A CustomDataDefinition itself comprises of four fields:

struct CustomDataDefinition {
  1: required string fieldName
  2: required string fieldType
  3: required string fieldDescription
  4: required string defaultValue
}

While the atom itself has an optional list of these:

8: optional list<CustomDataDefinition> customDataDefinitions = []

These will allow for a structured interface in Composer, with the description providing some context about the nature/use of the custom data and the default value ensuring a default/fallback behaviour.

[Fweddi]

string is the best we can do for defaultValue as Thrift doesn't support polymorphism.

I assume we will fill defaultValue with stringified JSON.

[gu-scala-library-release]

@Fweddi has published a preview version of this PR with release workflow run #23, based on commit 9082e3a:

7.0.0-PREVIEW.fpdefine-custom-fields.2024-12-16T1530.9082e3ad

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Click 'Run workflow' in the GitHub UI, specifying the fp/define-custom-fields branch, or use the GitHub CLI command:

gh workflow run release.yml --ref fp/define-custom-fields

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Click 'Run workflow' in the GitHub UI, leaving the branch as the default, or use the GitHub CLI command:

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[gu-scala-library-release]

@frederickobrien has published a preview version of this PR with release workflow run #75, based on commit 912a1f7:

14.0.0-PREVIEW.fpdefine-custom-fields.2026-08-19T1448.912a1f79

Want to make another preview release?

Click 'Run workflow' in the GitHub UI, specifying the fp/define-custom-fields branch, or use the GitHub CLI command:

gh workflow run release.yml --ref fp/define-custom-fields

Want to make a full release after this PR is merged?

Click 'Run workflow' in the GitHub UI, leaving the branch as the default, or use the GitHub CLI command:

gh workflow run release.yml

[michaelclapham]

Do you have any suggestions of what this fieldType will contain?
Are you planning for just string versions of JSON types? (object, array, number, string, boolean) ?

Perhaps add a comment to this thrift explaining what values you think could go into this field?

[michaelclapham]

You could go really crazy with fieldTypes if you wanted to do advanced things later in editorial tooling like

• date
• array
• int (not fractional numbers)
• int-range
• number-range
• dropdown of strings
• coordinates

These are future concepts deffo not for this first PR, but will having this field type as a string be good enough to enable that sort of stuff later or is it worth setting it up as an enum or something else now?

[jonathonherbert]

NB @michaelclapham that a broader discussion is ongoing @ https://github.com/guardian/flexible-model/discussions/103.