Update AthRNGSvc configuration method to standard syntax

This merge request updates the CA-based configuration of AthRNGSvc, so that the config method has a more standardised interface and adds explicit calls to the AthRNGSvcCfg method to the Cfg methods of client components, so that we don't always end up with the default instance (for example in the case that a random number generator other than Mersenne Twister is requested).

It also cleans up the configuration of the old thread-unsafe random number service instances used by Fatras components.

In addition to making the configuration cleaner these changes reduce the number of spurious differences seen when comparing old and new-style configuration pkl files.