feat: Github actions workflow to send a message to Zulip thread

[Alex-Zughaid]

Runs a Github actions workflow at 8am every day to get a summary of recent changes to the repo and any outstanding PRs.

Then use a Zulip bot to send a message to the 'Repo Updates' thread.

NOTE: needs GH secrets to be configured in repo settings to test this

[github-actions]

Thank you for this PR, which will now be reviewed. If submitting to ./Physlib or ./QuantumInfo, please see our review guidelines if you are not familiar with the process. You should expect a back and forth with a reviewer before your PR is merged. See also that link for how to add appropriate labels to your PR. The PR will also go through a number of automated checks. You can learn more about these here, including how to run them locally.

If you are submitting to ./PhyslibAlpha there will be a lighter review process, though your PR must still pass the automated checks.

If you want to bring attention to this PR, please write a message on this thread of the Lean Zulip.

Important: If a reviewer adds an awaiting-author label to your PR, once you have addressed the review comments, please remove that label by adding a comment with -awaiting-author. This helps us keep track of reviews.