Webservice: implement content_security_policy configuration

Summary

Implement the Content Security Policy (CSP) configuration for Webservice, which has been available since %12.2. This is additional configuration within gitlab.yml, which is not enabled by default

Current behavior

Unable to configure CSP for Rails

Expected behavior

Possible to configure CSP for Rails, and see it in action.

Versions

Related to gitlab-com/gl-infra/delivery#1065 (closed)