Draft: Add a feature flag use_base_class_in_proxy_util

What does this MR do and why?

Added a feature_flag use_base_class_in_proxy_util which simplifies the code inside the proxy_util to find the Config. Feature flag uses the base_class method. For all the ApplicationRecord classes .base_class always gives the deepest parent class which is inheriting from ApplicationRecord.

Note: For non ApplicationRecord base_class method is not defined. We have two classes Wiki and Repository which are non ApplicationRecord and they don't have base_class implemented. I don't think we should use respond_to? to avoid exceptions. If there is an exception we should implement the method base_class. For now, there will be no exception because use_separate_indices is false.

How to set up and validate locally

Just enable the feature flag and perform some global searching. And verify everything works as before.

Feature.enable :use_base_class_in_proxy_util

MR acceptance checklist

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