Add slot for externalAuthor

What does this MR do and why?

Query for externalAuthor on Service Desk issues list and add option for it to display in IssuableList component. I kept the convention for a current Service Desk list:by xxx@gmail.com via GitLab Support Bot

Backend for that new field: !129665 (merged)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

SERVICE DESK SETUP

1. Setup a service desk on your GDK: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/service_desk_mail_room.md
2. Send an email to your Service Desk, so you have something to display on the list.

ALTERNATIVE SETUP

1. You can omit Service Desk setup and force an already existing issue to become a SD issue. Open the rails console: rails c
2. Use these commands. Change 7 to the project id that matches the one you want to use for testing:

email = 'your-email@gitlab.com'
project = Project.find 7
issue = project.issues.last
issue.update!(service_desk_reply_to: email, author: Users::Internal.support_bot)
IssueEmailParticipant.create!(issue: issue, email: email)

1. In rails console enable the feature flag

   Feature.enable(:service_desk_vue_list)
   Feature.enable(:frontend_caching)

2. Visit the project that has Service Desk setup

3. In the main navigation click Monitor, than Service Desk.

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 #413092 (closed)