Add sync-upstream workflows

[guicaulada]

I think I'm in a good position with the sync workflow now, it needs some love fixing versions, removing test steps, etc. I expect a lot of feedback from the team to improve it. But I think the approach is good now.

We have 3 workflows:

• One responsible for creating the PR with the upstream changes.
  Currently running on my pull request on changes, in the future it will run on a cron or manually.
• One responsible for updating the PR, in case there are changes on main that weren't included on the PR we can request changes and the bot will rebase the latest main and update the PR.
• One responsible for pushing the changes to main, once the PR is approved the bot will try to push the changes to main.

I tried to merge the last 2 workflows but if we rebased just before pushing the changes from the latest rebase wouldn't be reviewed and conflicts could be caused.

If there are conflicts the PR should be created anyway, I don't know if we can create a empty PR but if we can't I will make a dummy commit just to create the PR and will advise on the body on how to proceed to rebase without the dummy commit.

There are a couple pull requests from the bot so you can see how they look, one of them simulates conflicts, the other doesn't:
https://github.com/grafana/runner-images/pulls

[CLAassistant]

All committers have signed the CLA.

[dsotirakis]

Good job! A few questions mostly for my understanding of things

[iainlane]

great, this is looking good 👍

mostly have smaller comments and questions now 😁

[iainlane]

looks great, just some tiny nits 👍 🚀 🥳 🎈 🎆