Add Gitlab:Observability.observability_enabled?

What does this MR do and why?

gitlab-org/opstrace/opstrace-ui#152 (closed)

Refactoring the logic that checks if observability is accessible and enabled for the current user / group out of the observability_controller and moved it into Gitlab:Observability lib, so that it can be accessed from different parts of the app

Screenshots or screen recordings

N/A

How to set up and validate locally

• Run rspecs
• https://gitlab.com/gitlab-org/opstrace/opstrace-ui/-/blob/main/contribute/developer-guide.md#run-gitlab-observability-ui-with-local-gdk