add suggestPhotonMapKeys() to TrigGlobalEfficiencyCorrectionTool

These suggest*() functions are auxiliary static methods of the class that can be used to help configuring an actual tool instance. Only electrons were supported so far, this extends to photons, for !64337 (merged). The MR adds two functions, the one from the the title and a more generic suggestEgammaMapKeys() that takes an extra argument (the desired object type).

The example 4 in the package was extended to test the photon implementation.