Add ShadowEvent flag

Merged John Derek Chapman requested to merge jchapman/athena:AddShadowEventFlag_23.0 into 23.0

This merge request just adds two sim flags, which will be used by the developments in !63059 (merged).

This should make no difference to the simulation output, but it is useful to add it first, so that the other changes can be added independently and be shown to also not change the simulation output.

Tagging @mbandier, @tlari, @averbyts, @pclark

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