Workflow to sync from upstream repo

[narendraReddy45]

Description & Motivation

Keeping this fork in sync with upstream Cloud-Foundations/Dominator has been a manual process. Someone has to remember to check for new commits, pull them in, and resolve any conflicts. This PR automates that with a scheduled sync workflow, so upstream changes get surfaced and merged (or flagged) on a daily cadence instead of tracked by hand.

JIRA: https://jira.purestorage.com/browse/PEFM-364

Approach

This PR implements the below:

• Runs a cron GitHub Action every day at 6 AM UTC (also triggerable manually via workflow_dispatch) to check upstream Cloud-Foundations/Dominator, diffing upstream's master against this fork's master. If a PR already exists, it's updated with a comment noting the latest changes. If not, a new PR is created. If no diff exists, the workflow exits without any modifications.
• The diff check also compares actual trees, not just commit count, so commits already squash-merged or cherry-picked into the fork don't trigger a no-op PR.
• If the merge would produce conflicts, the PR is opened or kept as a draft with a conflicts label and includes manual resolution steps, instead of being marked ready for review.
• Manual runs support two options: dry_run (detect and report changes without pushing a branch or opening a PR) and force_sync (open a sync PR even if the fork appears up to date).
• The whole workflow is gated behind an UPSTREAM_SYNC_ENABLED repo variable, so it's inert until explicitly turned on.
• Sends Slack notifications for created/updated PRs, and a separate notification when a sync PR is merged into master.

Testing

Tested on personal fork , sample PR created with diff commits
narendraReddy45#3