We are looking for a way to query an external API and populate the result in a grid in case view or on a task, depending on complexity.
Reading through The EDS documentation it sounds very restrictive to case fields.
If I have a custom widget with a grid, can I use EDS to call a REST API and populate the result into the grid? (Assuming that authentication is not a problem)
What you are probably looking for are external properties. Take a look at the documentation Knowledge Center - Case Managet: Using external properties