GLQL - search ES Work Items by id

The following glql query fetches a specific work item by it's iid, even though it's called as id it actually references iid field.

```glql
display: table
fields: title, labels, milestone
query: group = "gitlab-org" and id = 652
```

• We need to create a new by_iid ES filter in ee/lib/search/elastic/filters.rb
• and make sure to allow iid param in ee/app/finders/ee/work_items/glql/work_items_finder.rb