Enable authentification

[darya-mart]

• Add RequestManager class to managing HTTP requests with optional HTTP basic authentication.
• Minor changes: Fix the api path ("/" at the end of the path)
• Remove init params api_endpoint and port of BackendClient due to refacotring (are handled by RequestManager now )

Question: Has the code if port is None: \n port = 9010 a purpose? This was deleted first since the purpose is unclear.