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/models/tasks.go

@@ -176,6 +176,7 @@ type taskSearchOptions struct {
 	filter             string
 	filterTimezone     string
 	projectIDs         []int64
+	expand             TaskCollectionExpandable
 }
 
 // ReadAll is a dummy function to still have that endpoint documented