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

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.

• I have evaluated the MR acceptance checklist for this MR.

Related to #361748 (closed)