Add assignee dropdown to modal

What does this MR do and why?

This MR adds support for assigning an incident to a member of the selected project while creating incidents.

Adds support to search with query strings among project members.

If the user is not a project member, it will create an incident but no one would be assigned and a message is shown in the response. (Check the video for more details)

Screenshots or screen recordings

Screen_Recording_2023-01-23_at_12.55.43_PM

How to set up and validate locally

1. Follow the steps mentioned in https://gitlab.com/gitlab-org/manage/integrations/team/-/blob/main/integrations/slack.md to setup slack dev env.
2. Open the incident modal using /dev-gitlab incident declare
3. Search for a user from the Assignee dropdown, add a search string (minimum 3 letters) and see the suggestions with user names matching the search strings.
4. The suggestions will only be shown for the users matching search string and are a member of the selected project from the project dropdown.
5. Select a user and fill in the other details and create an incident.
6. Go to the incident details page and see that incident getting assigned to the user you selected.
7. If the user it not a member in the project, ( that is if they are guest), they won't be assigned to the incident.

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 #374967 (closed)