Extract Work item contents and create a wrapper

What does this MR do and why?

⚠ Note: This MR is part of the preparation work required for &7459 (closed), where we're extracting parts of the Hierarchy Widget frontend such that it can be re-used for Linked Work Items app.

As highlighted in the screenshot below, a single work item container which is shown in Hierarchy Widget will look exact same for Linked Work Items widget, including the metadata that is present within the container, so we need to create a component just for the contents of work item, such that it can be shared between both the apps (i.e. Hierarchy & Linked Items), additionally, any reusable parts of the app are required to be moved to shared folder within work items.

Following changes are made to the component structure within app/assets/javascripts/work_items/:

• Created components/shared/work_item_link_child_contents.vue by extracting contents from components/work_item_links/work_item_link_child.vue.
• Move components/work_item_links/work_item_link_child_metadata.vue & components/work_item_links/work_item_links_menu.vue to components/shared

Screenshots or screen recordings

No visual impact.

How to set up and validate locally

Prerequisites:

Enable the OKRs feature as mentioned in the OKRs docs.

Steps to verify:

1. Login with any user
2. Go to any Project > Issues > List
3. Create an new Objective under New issue split button
4. Go to Objective detail page
5. Add a Objective or Key result in the Child objectives and key results
6. Add few more so that the order of the list can be changed by dragging and dropping
7. Click on any item and other meta data like Labels, Milestone, Progress, Health status.
8. Check if all data are present on the list

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 #416558 (closed)