Validate "Unleash" client settings configuration in the gitlab-ci.yml file

The following discussion from !18606 (merged) should be addressed:

• @ayufan started a discussion: (+12 comments)

  I think that we should try to validate that configuration does have all items set, we will not be able to validate the URL, as it can be simply not accessible. This at least prevents some misconfiguration.

  The case of Object Storage is interesting. I don't know if we validate the completeness of the configuration (or if we can do it actually), but we validate the direct_upload option at least that it is in fact supported: config/initializers/direct_upload_support.rb.

  Yes, it is not enough, but at least you know that all required parameters are passed to the application if your intent was to enable feature flags. Otherwise, you have a silent failure and hard to find a reason why you cannot toggle feature flags (or you even do not notice that). This makes a support burden slightly smaller IMHO by hard checking the consistency of the configuration.

  Given that I think that I would prefer to have a hard failure if for whatever reason my intent (enable feature flags) cannot be fulfilled because I missed the config option that is required. Debugging this to notice that I did not pass instance_id means that you have to check for this type of information very deeply (look for particular information in a particular log file).