Sorting issues by label priority in graphQL

What does this MR do?

It adds the possibility to sort by label priority to the graphQL endpoint.

Requirement for #197227 (closed)

Related to #38295 (closed)

Screenshots

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.