Slack messages on each example test and the full workflow failure/start/success

[divyenduz]

This should already happen in some capacity via https://github.com/prisma/prisma-examples/pull/2180 https://github.com/prisma/prisma-examples/pull/2111 and https://github.com/prisma/prisma-examples/pull/2100

This issue tracks writing down the slack notification behaviour we actually want and implementing it.