API support for Preemptive Bearer Token

[afrimberger]

Current behavior

The UI allows setting the HTTP authentication method Preemptive Bearer Token for NPM proxy repositories:

However, the REST API currently only supports setting the auth type to username or ntlm:

See HttpClientConnectionAuthenticationAttributes

Expected behavior

The REST api endpoints for NPM repositories support the auth type "bearerToken".

How to solve?

Parts of the code already respect the authType=bearerToken. I will add an MR with some code demonstrating how to fix it.
I would be really happy if we can get these minimal adaptions upstream.