NEXUS-5849: Central prefix file is served through Artifactory, which results in non-central artifacts being blocked

Introducing blacklisted servers, that are able to "divert" automatic routing.

Change: basically "pulled up" HttpClientManager into a newly introduced support class, where all the shared code for handling HC4 is (used by RemoteScrapeStrategy so far). One method basically checks for "blacklisted" servers, used by both strategies.

Side effect: PropfileDiscoveryStatusSource fixed for resource handling.

Issue
https://issues.sonatype.org/browse/NEXUS-5849

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF38