Major updates and changes in the library

[danishjamal104]

Below are the changes this PR includes

1. Removed all the models.
2. Removed/fixed the model related areas.
3. Modified BaseApiManager and its implementation.
   1. Removed the userPreference property from implementation class, since it was only used for getting instance url and tenant viz now passed in create service method itself.
   2. Modified function signature, now each function returns NonNullable object.
   3. ConvertorFactory and CallAdapterFactory modified.
4. Mappers design modified
   1. Two more methods, mapFromEntityList and mapToEntityList added in EntityMapper interface.
   2. Created new class AbstractMapper, which implements the above two methods and open rest to be implemented accordingly.
   3. Now onwards for getting the generic behaviour all the mappers must extend the AbstractMapper and for custom logic implement EntityMapper.