News items list should include opportunities

[stefanprobst]

Is your feature request related to a problem? Please describe.

Currently, the news items list endpoint does not include opportunities.

Describe the solution you'd like

We need to include opportunities (those with source: "dariah" but not those with source: "external") in the list of news items.

Which means featured news items need to be able to select those opportunities as well.

Things to clarify:

• opportunities currently don't have an image field, but for news items it is a required field
• summary is required in news items but optional for opportunities
• opportunities don't have a publicationDate field, they have duration and updatedAt
• should we create a new endpoint for this combined list - leaning towards yes, because we don't want to have /news/:id or /news/slugs/:slug return an opportunity
• decision needed whether funding-calls should also be included in the news-items listing

Describe alternatives you've considered

No response

Additional context

see DARIAH-ERIC/dariah-website#325