External imports: Allow mapping of children object types in multiple import configurations

Problem

Currently External Imports and their mappings are designed for one import configuration per object type (root object type).

For instance if we have the the following structure:

and want to have 2 different import configurations for each of Toyota and Mazda object types, in the mapping of both imports, should provide the mapping for the root object type (Car) with all children. Even though, the import configuration for Mazda should not need to know about Toyota object type and vice versa. Otherwise, if only Mazda object type mapping is provided, Toyota object type will get deleted when the `POST .../importsource/{id}/mapping` endpoint is hit. 

Suggested Solution

Allow mapping of children object types in multiple import configuration so that each import configuration can only define the child object type they are dealing with and doesn't have to provide all children object types of the same parent. 

Why This Is Important

Flexibility of bringing data for children object types from different sources (through different import configuration).

Workaround

Currently each import configuration needs to provide all the hierarchy of the root object type in the mapping. Even the children object types that are irrelevant to the data that is being imported.