Add observability settings application setting

What does this MR do and why?

• Adds a new checkbox to enable/disable the periodic instance-level job to collect Observability errors from the cloud-connected GitLab Observability Backend.
• Stops those network calls from happening when disabled to avoid network errors piling up in the sidekiq logs.

References

• Customer bug report: Observability::AlertQueryWorker causing "Failed... (#496661 - closed) • Max Woolf • 17.8

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screen recording of reproduction: https://gitlab.zoom.us/rec/share/5xrFvI3gHjxIJBjd9DwvlzmcoEudAeJ19hM2LIpyPZCJXMELFHON---bZIMOgkfk.n3RCLedETcTceUf3?startTime=1734600834000

How to set up and validate locally

• Enable the observability_features feature flag.
• Login as an instance admin, and visit Admin Area -> Network -> GitLab Observability Backend requests.
• Try to toggle the new Fetch cloud observability alerts checkbox, and ensure the result is persisted to the database.
• Toggle the flag off.
• Block your computer's access to observe.staging.gitlab.com. You could add something like 182.1.1.34 observe.staging.gitlab.com to /etc/hosts where the IP is some unreachable or made-up IP address.
• Run the worker locally, it should timeout with Net::OpenTimeout.
• Toggle the admin flag on.
• Run the same worker, and it should exit immediately after the licence check.

Related to #496661 (closed)