Add a flag for enable/disable observability alerts to project settings

What does this MR do and why?

• Adds a column observability_alerts_enabled to project settings. Rationale is to allow users to disable alerts created via observability errors. See Observability alert creation via a sidekiq worker (!162068 - merged) for the worker addition.
• Updates the observability specific project settings section to use rails + haml.
• Adds a form on the observability settings section to allow users to enable/disable alerts.
• Updates the ee/search/project_settings to add the new observability settings section.
• Followup for adding a frontend specific spec to validate settings form is here: gitlab-org/opstrace/opstrace#2942 (closed).

Related to https://gitlab.com/gitlab-org/opstrace/opstrace/-/issues/2813.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Setup GDK and run the associated migration

   VERSION=20240829101214 bundle exec rails db:migrate:main

2. Make sure observabilty feature flag is enabled on a group (root namespace) and project alongside a EE license.
3. Visit Settings > Monitor tab from the navigation for an available project.
4. Click Enable Observability Alerts button.