Scaffolding Observability Tracing feature

What does this MR do and why?

First batch of changes to introduce the new Observability Tracing feature. (docs)

• Add tracing feature flag (rollout issue)
• Add Monitor > Tracing nav menu ( #416655 )
• Set up basic tracing route/controller
• Make observability CSP concern more generic, by removing previous observability feature flag check.
• Added connect-src to the CSP rules, as it's going to be needed to make API calls to the observability backend

Frontend changes: !125014 (merged)

Closes gitlab-org/opstrace/opstrace#2238 (closed)

Screenshots or screen recordings

Before (FF off):

After (FF on):

How to set up and validate locally

• Rspecs

MR acceptance checklist

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