ITrigEventLoopMgr: add prepareForStart method

Add a new `ITrigEventLoopMgr::prepareForStart` method to the
`HltEventLoopMgr` that is being called by the `Psc` before calling the
start method of the `ApplicationMgr`. This will allow the EventLoopMgr to
setup anything that may be required during `start` (e.g. setting up
a partial EventContext, ATR-21331).