Add round-robin issue assignment workflow

[fabiocav]

This pull request introduces a new GitHub Actions workflow to automate round-robin assignment of issues when they are opened. The workflow ensures that each new issue is assigned to the next person in a predefined list, and it persists the last assigned index to maintain the rotation order, even if multiple issues are opened in quick succession.

Pull request checklist

IMPORTANT: Currently, changes must be backported to the in-proc branch to be included in Core Tools and non-Flex deployments.

• Backporting to the in-proc branch is not required
  • Otherwise: Link to backporting PR
• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• I have added all required tests (Unit tests, E2E tests)

[Copilot]

Pull request overview

Adds a GitHub Actions workflow to automatically assign newly opened issues using a round-robin rotation, persisting state between runs.

Changes:

• Introduces an issues: opened workflow that selects the next assignee from a predefined list.
• Persists the last-used index in a GitHub Actions repository variable to maintain rotation order.
• Serializes workflow executions via concurrency to reduce race conditions when multiple issues are opened close together.