Extend BaseHttpClient and ApiExecutor to process requests with multipart/form-data

Desired Behavior

The newly introduced BaseHttpClient class makes use of the java.net.http.HttpClient class. Multipart Form Data, e.g. needed for import/upload, is handled differently than in the formerly used Apache HttpClient. 

The new BaseHttpClient should be able to process Multipart Form Data. The ApiExecutor should make use of this new capability to be able to handle file uploads, e.g. for the JS7 JOC API import.