Make GraphQL epics connection countable

What does this MR do and why?

MR adds count to the existing EpicConnection type so that GraphQL API consumers could retrieve the total count of epics. This MR implements only the GraphQL part of the original related issue (#361748 (closed)).

Screenshots or screen recordings

Before/after screenshots
Before After
96776-error 96776-successful

How to set up and validate locally

  1. Open the GraphQL client (e.g. /-/graphql-explorer on local GDK)
  2. Run the following query
query groupEpics($groupPath: ID!) {
  group(fullPath: $groupPath) {
    id
    epics (first: 2) {
      count
      nodes {
        id
        iid
        title
      }
      pageInfo {
        hasNextPage
        hasPreviousPage
        startCursor
        endCursor
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #361748 (closed)

Edited by Stanislav Dobrovolschii