Deprecate get_user_metadata and apply_user_metadata methods in BaseRegistry

[tokoko]

We have get_user_metadata and apply_user_metadata methods in BaseRegistry that I doubt serve any real purpose. It can only be used for feature view, while users are also free to use tags instead for all feast types. The main difference is that tags are textual, while user_metadata stores bytes.

I think we should deprecate and eventually remove the abstract methods from BaseRegistry.