feat(tasks): expand subtasks (#2345)

This change adds a parameter to expand subtasks - if provided, Vikunja will ensure all subtasks are present in the results list.

Resolves https://community.vikunja.io/t/subtasks-show-on-different-pages/2292
Reviewed-on: https://kolaente.dev/vikunja/vikunja/pulls/2345
Co-authored-by: kolaente <k@knt.li>
Co-committed-by: kolaente <k@knt.li>

pkg/swagger/swagger.yaml

@@ -3336,6 +3336,14 @@ paths:
         in: query
         name: filter_include_nulls
         type: string
+      - description: If set to `subtasks`, Vikunja will fetch only tasks which do
+          not have subtasks and then in a second step, will fetch all of these subtasks.
+          This may result in more tasks than the pagination limit being returned,
+          but all subtasks will be present in the response. You can only set this
+          to `subtasks`.
+        in: query
+        name: expand
+        type: string
       produces:
       - application/json
       responses: