Add way of getting custom model replacements like FindSoundReplacementSample()

[incognico]

Add way of getting custom model replacements analog to FindSoundReplacementSample().

For example:

string FindModelReplacement(CBaseEntity@ pEntity, const string& in szModelName) const

Finds the model replacement file/path for the given entity and model. If the the model is replaced, it will return the replacement model file/path instead. Otherwise, szModelName is returned.