Add basic components for Tracing UI

Initial round of frontend changes for the upcoming (feature flag-ed) Tracing UI (backend changes: !124966 (merged))

• Create reusable ObservabilityContainer component
  • It handles the auth flow with the Observability Backend (observe.gitlab.com - Note this is not Rails) by injecting an iframe into the page, and later remove it upon auth completion (more details in the related issue)
  • It builds the ObservabilityClient (currently mocked as backend API is still WIP) and injects it into its child components. Every page that needs to be able to interact with the observability backend, can just be wrapped in the ObservabilityContainer, and will get auth and API client setup for free.
• Setup basic view ( more UI changes coming separately to reduce the scope of the MR. !125572 (merged) )

Closes gitlab-org/opstrace/opstrace#2248 (closed)

Screenshots or screen recordings

Nothing to see (yet)

How to set up and validate locally

• rspec

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.