Graphql mutation for (un)subscribing to an epic

What does this MR do?

It adds a mutation for (un)subscribing to an epic.

Screenshots

Mutation Example:

# query
mutation epicSetSubscription($groupPath: ID!, $iid: ID!, $subscribedState: Boolean!) {
  epicSetSubscription(input: {groupPath: $groupPath, iid: $iid, subscribedState: $subscribedState}) {
    errors
    epic {
      subscribed
    }
  }
}

# variables
{ "groupPath": "twitter", "iid": 1, "subscribedState": true }

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Issue #34132 (closed)