Add Slack notification to healthcheck

[KevinMind]

Fixes: mozilla/addons#15440

Description

• Updated the health check workflow to generate health check output and message JSON files.
• Introduced a new script, health_check_blocks.py, to format health check data for Slack notifications.
• Modified health_check.py to support output file generation and improved error handling.
• Added tests for the new health check block formatting functionality.
• Updated the workflow to send notifications to Slack with health check results, excluding local environment notifications.

Context

This enahnces the healthcheck to add slack notifications when there are failures and a notification if the workflow fails for any reason.

Testing

You can run the healtcheck locally, modifying monitors to get different results. set an --output property to save the file.

health_check_file="health_check.json"
health_check_blocks_file="health_check_blocks.json"
env="local"
./scripts/health_check.py --env $env --output $health_check_file --retries 0
./scripts/health_check_blocks.py --input $health_check_file --out $health_check_blocks_file

Then you can execute the bash script line by line to se what the final payload to slack would look like..

Finallly you can execute the healthcheck via github actions on this branch

gh workflow run health_check.yml --ref kevinmind/addons/15440

This PR introduces a temp failing monitor that should trigger a notifaction to the channel.

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.