Adding Observability CSP concern

What does this MR do and why?

gitlab-org/opstrace/opstrace-ui#144 (closed)

We need to extract the logic that handles CSP rules for Observability UI (GOUI) from the Observability Controller and make it reusable for controllers/routes where we want to be able to embed the GOUI. Such logic can be extracted and reused as an ActiveSupport::Concern

This change is needed to enable embedding the GOUI in other parts of GitLab, such as issues and MRs (gitlab-org/opstrace&33)

Screenshots or screen recordings

N/A

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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