[HLT] Move online error monitoring from event loop manager to a dedicated tool

Follow-up from a discussion in !36901 (comment 3830204)

Move the method algExecErrors from HltEventLoopMgr to a new dedicated tool in the TrigSteerMonitor. The new tool, TrigErrorMonTool will be expanded with a new histogram in !36901 (merged). The move is needed to minimise library dependencies in TrigServices.

Moving this to TrigSteerMonitor has the following advantages:

• the tool creates framework monitoring histograms, so functionally this is the right package for it
• there is already a header-only library TrigSteerMonitorLib which TrigServices already depend on (for SchedulerMonSvc¹)
• the TrigSteerMonitor component library already depends on everything that will be needed for the error histograms and contains other monitoring components having similar functions (e.g. the TrigSignatureMoniMT)
• this is exactly where the same functionality for Run-2 was implemented - the old tool is still there, TrigErrorMon (similar name, the old code to be removed before Run 3)

cc @aporeba, @fwinkl, @tamartin, @wiedenma, @mark

¹ SchedulerMonSvc may get moved somewhere else at some point, but we'll see about that. Still, this header-only library seems like a good solution to let HltEventLoopMgr use the external tool with minimal dependencies.