Policies: Normalise the use of policies schema and its types

Policies from the Apicast registry, policy chain and custom policies should be using the same types for its properties. Also, manage the schema the same way. Right now it's treated different and it's confusing.
We should base our refactor on the latest Custom Policy definition since it's more solid. How to manage the schema and it's types.

Dev notes:

We are confident enough on our test to make this refactoring. There is no channges foreseen in Ruby nor in the behaviour

It will take some time, many concepts, attributes and uses are quite tangled.