Manage and display labels from epic graphql

What does this MR do?

This MR adds labels field to the EpicType and adds the possibility to add and remove labels in UpdateEpic mutation - new arguments add_label_ids and remove_label_ids were introduced, which is the similar approach as we use currently in services.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• 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.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

Related issues: #35185 (closed), #31912 (closed)