Disable implicit pool creation on link behind a feature flag

This commit adds a feature flag to disable the implicit pool creation when joining a repository to a non-existent object pool. It looks like this behavior is not needed given Rails should create the object pool prior to considering it ready for joining. Disabling this behavior makes it easier to handle pools in Praefect as Praefect needs to create metadata records for all of the repositories. This implicit behavior might create a repository without Praefect's knowledge. Special handling would be required for this particular RPC only, so let's see if we can remove the behavior rather than implement it in Praefect.

Related to: #4089 (closed)