feat: add upstream sync workflow

[linvald]

This PR adds a GitHub Actions workflow to automatically sync with the upstream repository alexandrevilain/temporal-operator.

Features

• Scheduled trigger: Runs weekly on Monday at 6 AM UTC
• Manual trigger: Can be triggered manually via workflow_dispatch
• PR-based sync: Creates a PR for review rather than auto-merging changes
• Change detection: Only creates a PR when there are actual upstream changes

Workflow Details

The workflow:

1. Fetches changes from the upstream alexandrevilain/temporal-operator repository
2. Compares the local main branch with upstream
3. If changes are detected, creates a PR titled "chore: sync with upstream temporal-operator"
4. The PR is labeled with upstream-sync and automated for easy filtering

This allows the team to review upstream changes before merging them into our fork.