Add IRepo interface

[yev-kanivets]

IRepo interface will represent a contract of access to abstract storage with CRUD operations:

interface IRepo<T> {
    T create(T instance);
    T read(long id);
    T update(T instance);
    boolean delete(T instance);
    List<T> readAll();
    List<T> readWithCondition(String condition, String[] args);
}