Add allowsMultipleAssignees field to assignees widget defintion

What does this MR do and why?

New EE only field to expose if licensed feature multiple_issue_assignees is available for the group or project exposing the list of work item types

How to set up and validate locally

Go to GraphiQL locally at http://localhost:3000/-/graphql-explorer and test the following queries

Group level

{
  group(fullPath: "flightjs") {
    workItemTypes {
      nodes {
        id
        name
        widgetDefinitions {
          type
          ... on WorkItemWidgetDefinitionAssignees {
            allowsMultipleAssignees
            canInviteMembers
          }
        }
      }
    }
  }
}

Project level

{
  project(fullPath: "flightjs/flight") {
    workItemTypes {
      nodes {
        id
        name
        widgetDefinitions {
          type
          ... on WorkItemWidgetDefinitionAssignees {
            allowsMultipleAssignees
            canInviteMembers
          }
        }
      }
    }
  }
}

Work item level

{
  workItem(id: "gid://gitlab/WorkItem/<gid>") {
    id
    namespace {
      fullName
    }
    workItemType {
      name
      widgetDefinitions {
        type
        ... on WorkItemWidgetDefinitionAssignees {
          allowsMultipleAssignees
          canInviteMembers
        }
      } 
    }
  }
}

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 #431492 (closed)