Add Objective and Keyresult to work item types

What does this MR do and why?

Adds two new work-item types(Objective & Key Result) to support OKRs tracking in Gitlab

Technical discussion link - gitlab-org/incubation-engineering/okr&2 Implementation issue link - gitlab-org/incubation-engineering/okr/meta#4 (closed)

Screenshots or screen recordings

How to set up and validate locally

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

MR acceptance checklist

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