Add edit to work item assignees

What does this MR do and why?

This MR makes changes to the already existing reusable component WorkItemSidebarDropdownWidgetWithEdit to support multiselect and single select.

Changes have also been supported in already existing components. Thank you !

Add edit to work item assignees

:Behind work_items_mvc_2 FF

MR acceptance checklist

• Infinite scroll should work when added new members by Invite members footer button
• search should work as expected
• single select should close the dropdown and apply the value
• clear should clear all the values
• multi select values should be applied by clicking anywhere outside the dropdown

Screenshots or screen recordings

To test multiselect/single select feature please apply the following patch

false for single select

true for multi select

diff --git a/app/assets/javascripts/work_items/components/work_item_attributes_wrapper.vue b/app/assets/javascripts/work_items/components/work_item_attributes_wrapper.vue
index 76f1938c3ed8..c3ce307335e0 100644
--- a/app/assets/javascripts/work_items/components/work_item_attributes_wrapper.vue
+++ b/app/assets/javascripts/work_items/components/work_item_attributes_wrapper.vue
@@ -124,7 +124,7 @@ export default {
         :full-path="fullPath"
         :work-item-id="workItem.id"
         :assignees="workItemAssignees.assignees.nodes"
-        :allows-multiple-assignees="workItemAssignees.allowsMultipleAssignees"
+        :allows-multiple-assignees="false"
         :work-item-type="workItemType"
         :can-invite-members="workItemAssignees.canInviteMembers"
         @error="$emit('error', $event)"

Single select	multi select
single select video	multi select video

How to set up and validate locally

1. Enable FF work_items_mvc_2
2. Go to any work item -> task/key result/objective
3. See that you are able to apply the single select/multi select value as shown in the video

Related to #429133 (closed)