This PR migrates the http client implementation to be based on httpclient5.
There could be plenty of optimizations as httpclient5 brings some very cool features, however this is not the goal of this PR.

One thing I want to point out is that the test classes CloseableHttpClientStub and CloseableHttpResponseStub had to be moved to another package in order to allow them accessing methods of httpclient5 that are package protected.

Testing done

Ran mvn clean verify

Submitter checklist

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