Add `include_descendant_groups` to Epics GraphQL

What does this MR do?

Related to: #250281 (closed)

With this argument one can filter Epics GraphQL API to return only the Epics direct on the given Group, the same way it's done in the HTTP API.

This will be necessary to Import Epics using the Group Migration (BulkImports) feature - !46710 (merged).

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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