Add support for adding/creating Key Results

What does this MR do and why?

Adds frontend support for creating and adding Key Results within an Objective.

Screenshots or screen recordings

Add Dropdown (Before)	Add Dropdown (After)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Open rails console by running bin/rails console within GDK directory.
2. Enable the feature flag :hierarchy_db_restrictions on a project; Feature.enable(:hierarchy_db_restrictions, Project.find_by_full_path('gitlab-org/gitlab-test')).
3. Enable the feature flag :okrs_mvc on the same project; Feature.enable(:okrs_mvc, Project.find_by_full_path('gitlab-org/gitlab-test')).
4. Visit issues list page for the project by navigating to http://gdk.test:3000/gitlab-org/gitlab-test/-/issues.
5. Create an objective by clicking on dropdown button next to New issue and selecting New objective from the dropdown.
6. Once objective is created, click on it to visit its Work Item page, you should be able to see Tree View below the description.
7. Use Add dropdown button create/add key results.

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.

Related to #381872 (closed), #381833 (closed), #385084 (closed)