Add avatar to new achievement form

What does this MR do and why?

This MR adds the field to upload an avatar when creating a new achievement

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screen_Recording_2024-03-11_at_01.17.16

How to set up and validate locally

1. Enabled the ff: Feature.enable(:achievements)
2. Visit the group achievements page (no nav menu item currently exists): http://gdk.test:3000/groups/flightjs/-/achievements/
3. Select New achievement
4. Select Choose file...
5. Select an image
6. Enter a name (and optionally a description)
7. Select Save changes
8. Check the avatar was uploaded via the rails console Achievement.last.avatar_url or graphiql:

   query {
     group(fullPath: "flightjs") {
       achievements {
         nodes {
           id
           name
           description
           avatarUrl
         }
       }
     }
   }