Expose blocking/blocked epics properties on GraphQL

What does this MR do and why?

Exposes related epics blocking/blocked information on GraphQL, we are adding the same properties that are already exposed for IssueType.

related to #355739 (closed)

How to set up and validate locally

Ultimate license required

1. Create an epic
2. Go to linked epics section
3. Add one epic blocking this epic
4. Add another epic blocked by the current one
5. Use [GraphiQL explorer] and try the following query for the epic you just modified:

query epicBlocksData {
  group(fullPath: "the epic group") {
    epic(iid: EPIC_IID) {
      blocked
      blockingCount
      blockedByCount
      blockedByEpics {
        nodes {
          title
        }
      }
    }
  }
}

MR acceptance checklist

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