Investigate changes to FederatedStoreCache.java

[GCHQDeveloper314]

Prior to #3147, FederatedStoreCache.java was inheriting from Cache.java solely to enforce some generics. It had various overriding methods which were unused but were left in place in that PR.

Either this inheritance should be put back, another way to fix the generics found or this class and FederatedStoreCacheTransient.java refactored to simplify them.