Add support for work items API in GLQL

What does this MR do and why?

Add support for work items API in GLQL

Behind a feature flag glql_work_items, we now support the work items API

References

• gitlab-org/gitlab-query-language/glql-rust#46
• gitlab-org/gitlab-query-language/glql-rust!114 released under version v0.5.0

Screenshots or screen recordings

No change in UI, except that the work items query is used instead of issues.

There are still many compatibility issues with the legacy API, which is why this is behind a separate feature flag. Issues like

• NOT filters don't support weight and health status (needs a fix in our backend).
• Health status doesn't work with work items (needs a fix in the compiler).

How to set up and validate locally

1. Enable feature flag :glql_work_items.

2. Embed the following GLQL query in an issue's description:

   ```glql
   display: table
   query: label != "foo"
   fields: title, created, author, assignees, updated, labels, weight, epic
   ```

3. The query should work as expected (the GLQL view loads).

4. Inspect the Network tab for an /api/glql request. It should send a call to fetch workItems instead of issues.

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