Add prometheus proxies

What does this MR do?

This MR adds prometheus proxies for group and project clusters. This will be used for cluster dashboard metrics similar to how we currently do environment metrics. These proxies are exposed via the routes

/groups/*group_id/-/clusters/:id/prometheus/api/v1/*proxy_path(.:format) /*namespace_id/:project_id/clusters/:id/prometheus/api/v1/*proxy_path(.:format)

Note that because of our shared cluster context, there is also a route /admin/clusters/:id/prometheus/api/v1/*proxy_path(.:format) that will not be used.

This work is a part of gitlab-ce#63044

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team