Add xAODMaker::ROOTHeaderLoaderSvc to HLT Jobs, master branch (2020.07.22.)

Introduced a new (simple) service for hacking around the issue discussed in ATR-21753 and https://sft.its.cern.ch/jira/browse/ROOT-10940.

The service just does one thing. It loads a list of headers using TROOT::ProcessLine(...). Exactly as @ssnyder described it as a workaround in:

https://sft.its.cern.ch/jira/browse/ROOT-10940?focusedCommentId=109626&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-109626

I'm still working on adding the necessary configuration to all the relevant jobOptions. But wanted to open the MR already in this state, just to show that a fix is on the horizon.