Introduce client manager base class

Each manager class is initialized from the object, which means that some logic, like endpoint defenition, is duplicated over all managers.

In order to solve that, and also efficiently refactor endpoint defenition for being a property, base manager class is being introduced.

Regarding the endpoint defenition, right now it is constructed statically on client init. While this does work, it may result in an unexpected behavior, when client is being re-used.

To be more error-prone, it’s construction moved to a property.