RestPerfClient: Add support for authentication.

It is not possible to use RestPerfClient for performance testing of subsystems that require authentication because there is no support for supplying credentials along with the requests. One example of a test that requires authentication is https://git.opendaylight.org/gerrit/#/c/30484. It will be able to run in only job but not in all job because that one pulls in authentication.

Tested with recent version of Beryllium (from before about 3-5 days) and found this part missing.