Make new-issue-dropdown more reusable

What does this MR do and why?

This moves the new-issue-dropdown component to the vue_shared/ directory and changes it slightly to make it more reusable. Most notably, the Apollo query is made configurable so that we can retrieve projects from outside of a given group. For now, these changes aren't being leveraged, but they will be in follow-ups.

The reasoning behind this is that we are trying to migrate the "New issue" dropdown away from select2 in the instance-level issues dashboard: https://gitlab.com/dashboard/issues. We should be able to piggy back on the existing group-level component once it's a bit more flexible. A bit more context can be found here: #374098 (comment 1242472780).

Screenshots or screen recordings

No visual change.

How to set up and validate locally

Currently, the new-issue-dropdown is visible in a group's issues list: :group > Issues > 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 #374098 (closed)