Add autocompletion endpoints for iterations

What does this MR do and why?

Related to #324298 (closed).

This MR sets up new autocompletion endpoints for iterations that will be used by the frontend client.

How to set up and validate locally

1. Prepare iterations. Visit any group and navigate to Issues -> Iterations in the left sidebar.

• Create a new iterations cadence by clicking New iteration cadence.
  • For simplicity, uncheck Enable automatic scheduling
  • To the created iterations cadence, add an iteration.
    • Create a past iteration (due date is in the past) => closed iterations should NOT show up in the autocompletion list.
    • Create an open iteration (start date is in the future)

2. Visit the group autocompletion endpoint

http://gdk.test:3000/my-group/-/autocomplete_sources/iterations

3. Visit the group's project(s) autocompletion endpoint

http://gdk.test:3000/my-group/my-project/-/autocomplete_sources/iterations

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.