Tezt/Cloud: introduce Alert_manager module and use it for the DAL scenario

What

Why

How

Manually testing the MR

dune exec tezt/tests/cloud/main.exe -- dal cloud --grafana --monitoring --prometheus --website -v --log-file log-am --alert-handler low-DAL-attested-ratio-slack-handler --dns-domain phink-test.nl-dal.saroupille.com --proxy --slack-api-url <your webhook url>

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR