Extract token selector input and create a shared component

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 seen in below screenshot, the highlighted part is the token selector input. It is extracted from existing hierarchy widget so that it can be reused in work item relationship widget.

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

• Created /shared/work_item_token_input.vue by extracting contents from /work_item_links/work_item_links_form.vue.

Screenshots or screen recordings

No visual changes.

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 an existing Objective or Key result in the Child objectives and key results
6. Check if all data are present

This can be similar observed for Issues detail page > Task section > Add task.

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)