Add callout to capi thrift model

[marjisound]

What does this change?

This PR updates the capi model to introduce the callout element type.

This PR is the fifth step of the work for adding a new callout element in CAPI pipeline, as explained below:

1. flexible-model: Add callout element to flexible content thrift model https://github.com/guardian/flexible-model/pull/52
2. flexible-content: Consume callout version of flexible model https://github.com/guardian/flexible-content/pull/4233
3. content-api/elasticsearch: Add callout element type to elasticsearch https://github.com/guardian/content-api/pull/2708
4. content-api/porter: Add callout element type to porter https://github.com/guardian/content-api/pull/2704
5. content-api-models: Add callout to capi thrift model (current PR)
6. content-api/concierge: update content-api-models version to add callout in concierge https://github.com/guardian/content-api/pull/2705
7. content-api-scala-client: Update capi model version Update capi model version content-api-scala-client#372

How to test

How can we measure success?

Have we considered potential risks?

Images

Accessibility

• Tested with screen reader
• Navigable with keyboard
• Colour contrast passed
• The change doesn't use only colour to convey meaning

[marjisound]

Create new Callout model in CAPI Concierge

[JustinPinner]

This looks good to me 👍